In this publication the authors describe the grassland communities rich in species especially those which occur on basic soils. The investigation was done in the Belgian part as well as in the Dutch part of St. Pietersberg situated south of the Dutch town of Maastricht. See map – fig. 1. St. Pietersberg is a long and narrow remnant of a plateau which becomes larger going towards the south. On the east side St. Pietersberg is bordered by the river Maas and on the west side by the rivulet Jeker. The altitude of the area investigated varies from about 100 m above sea level in the north till about 150 m above sea level in the south. The vegetation relevees were made on the basis of the principles of the French-Swiss school of phytosociology (Braun-Blanquet, 1964). In the middle of every plot the pH was measured at a depth of ca. 5 cm in the soil by means of a pH-meter with color indicator solution (Hellige). The calcium carbonate content was determined by means of a 10% HCl-solution (Soil Survey Staff, 1954). These measurements are recorded at the top of the table of phanerogams (Table I). During some days in august 1970 the course of temperature was measured at different depth levels in the soil on two localities with different exposition. See fig. 8. The subsoil of St. Pietersberg consist of chalk deposits from the Upper-Cretaceous. On the slopes these chalk deposits reach the surface or come very near to it. At the moment the chalk is in exploitation in open quarries of gigantic sizes, which destroy the landscape and the scenery. On top of the chalk tertiary and quarternary sand and gravel deposits are to be found. The climate has continental caracteristics. The precipitation is relatively low compared with the rest of the Netherlands, namely ca. 650 mm a year. In July and August the amount of precipitation is rather high, ca. 145 mm, whereas the months of February, March, and April are rather dry with only 125 – 140 mm. The average number of hours of sunshine is 1500 a year. Botanists have been interested in St. Pietersberg for a long time; for instance already in 1821 Bory de St. Vincent published a list of 627 phanerogams and 18 ferns, all growing on St. Pietersberg. In the present paper the authors pay attention to the botanical data of former times in so far as they are related to plants which appear in grassland communities rich in species. From this it becomes clear that several species have decreased or even totally vanished, such as Antennaria dioica, Sesleria caerulea, Viburnum lantana, Juniperus communis, and especially many species of orchids, e.g. Himanthoglossum hircinum, Anacamptis pyramidalis, Orchis simia, and Ophrys apifera. Old data concerning the Bryophyta and Lichens are less numerous and because of this it is difficult to assess whether species have decreased or even have vanished. Phytosociological data about the chalk grasslands and other grasslands rich in species hardly ever occur in literature. A well detailed study of the vegetation of St. Pietersberg such as the one in this paper has not yet been published. During the investigation, the field work of which was carried out in the summer of 1970, most attention was paid to the grasslands on basic and neutral soils. In the system of vegetation classification according to the French-Swiss school of phytosociology these vegetation belong to the alliance Mesobromion (Br.-Bl. & Moor 1938) Oberd. 1957; order Brometalia (W. Koch 1926 n.n.) Br.-Bl. 1936; of the class Festuco – Brometea Br.-Bl. & R. Tüx. 1961. Three variants are to be distinguished within the Mesobrometum erecti Scherr. 1925 emend. Oberd. 1957, subassociation typicum: a. Melilotus albus-variant (relevees 1-10) This community occurs only near the western entrance of the Jekertunnel on a steep S.W. facing slope which originated artificially ca. sixty years ago when the tunnel was constructed. Locality F – fig. 1. Here the chalk comes to the surface and because of the steep slope there is a constant sliding down of the surface material of the soil, which prevents soil maturation. Consequently, species from the alliance Onopordion acanthii Br.-Bl. 1926 occur in addition to the chalk grassland vegetation of this locality. The Onopordion acanthii is typical for such situations. Character species are a.o. Verbascum nigrum and Melilotus albus. See Table I. This locality is the only one on St. Pietersberg where Bromus erectus is to be found in such large quantities. b. Viola hirta – Carex flaccavariant (relevees 11-24) This is the variant the richest in species within the Mesobrometum erecti of St. Pietersberg. This applies both for phanerogams and cryptogams. This community is met with on steep S.E. facing slopes (21 – 47°) of the erosion canyons and on very dry places with much wind exposure. Locality C and E – Fig. 1. The variant is a.o. characterised by species which can be considered as faithful for the whole N.W. European area of the Mesobromion, e.g. Anthyllis vulneraria, Cirsium acaulon, Epipactus atrorubens, and Helianthemum nummularium ssp. nummularium. See Table I. The richness of cryptogams is striking, especially of thermophytical and xerophytical species, such as Abietinella abietina, Campthothecium lutescens, Encalypta streptocarpa, Pleurochaete squarrosa, and Toninio coeruleonigricans. See Table II. Over thirty species of mosses and lichens have been found in this type. c. Inops – variant (relevees 25 – 41) A number of species which often occur in the previous variant are absent in this community. On the Dutch part of St. Pietersberg the Inops – variant is the only community belonging to the Mesobrometum erecti which is to be found there. See Fig. 1 – locality B, C, D, and E. Furthermore, in this variant several species preferring some degree of manuring and higher soil moisture are penetrating, e.g. Arrhenatherum elatius, Poa pratensis, and Elytrigia repens. The following plant community, the Rumex acetosa – Brachypodium pinnatum-vegetation, doesn’t belong to the Mesobrometum erecti, but to the association Arrhenatheretum elatioris Br.-Bl. 1919, alliance Arrhenatherion elatioris Br.-Bl. 1925, and class Molinio – Arrhenatheretea R. Tüx. 1937. This is based on the frequent presence of character species of the Arrhenatherion elatioris such as Arrhenatherum elatius, Dactylis glomerata, Festuca pratensis, and Rumex acetosa. See Table I. Because of the occurrence of a number of characteristic species of the Mesobromion in this Rumex acetosa – Brachypodium pinnatum-vegetation this community has been taken into account in this study. Distribution: Fig. 1 – locality A, B, D, and E. It is for the same reason that also grasslands rich in species on acid gravel deposits of pleistocene origin on the plateau of St. Pietersberg have been investigated. A new association, the Brachypodio – Sieglingietum, has been distinguished. See relevees 54 – 72 on table I and II. The Brachypodio – Sieglingietum belongs to the alliance Violion caninae Schwick, (1941) 1944 em. Preising 1949, class Nardo – Callunetea Preising 1949. See Fig. 1 – locality D, F, and G. All the vegetation communities being described here are without treatment of manure. Until World War II they were pastured by sheep, afterwards they remained untouched. At the moment they burn down practically every year early in spring. See Fig. 4. The occurrence of Mesobromion character species on St. Pietersberg on moist soils most likely is due to the constant manegement of the grasslands for centuries in which no use was made of manure. In spite of the great culture-historical and biological value of the area, almost the entire St. Pietersberg is very seriously destroyed by the quarries mentioned above. This exploitation is carried out in the Belgian as well as in the Dutch part of St. Pietersberg. There is only one locality (B) mentioned in this publication, namely the Popelmondedal, which for the time being is protected by law. It can be said that the chalk grassland vegetation of St. Pietersberg, from a botanic-ecological and phytogeografical point of view, should be reckoned to the most interesting Mesobromion communities of N.W. Europe.