Microthamnion strictissimum Rabenh., recorded as a new species for the Netherlands by Koster (1963), has, however, been found before by Romijn (1923), Beijerinck (1927), and Middelhoek (1950). The related M. kützingianum is known from this country since 1887 (Weber-Van Bosse). The present author mentions 5 new localities in De Peel (a dead moorland region in Noord-Brabant and Limburg) and one in the Bergvennen (Overijsel). The following conclusions have been drawn from the study of the material: 1. In De Peel specimens are found, which doubtless belong to M. strictissimum or kützingianum, but intermediate forms are also met with; a further revision of the genus seems to be necessary. 2. M. strictissimum is ecologically well-defined. It prefers an environment, which, originally oligotrophic, has gradually changed into mesotrophic; mostly the bottom is sandy, but with a thin layer of organic material. In all localities where the species was observed the waterlevel had previously been lowered by drainage. 3. Two observations are not in accordance with this description of the environment: the record of Romijn, unfortunately impossible to be verified, and one of the records of Beijerinck, which probably concerns an other species.