Sea-grasses are phanerogams which are completely adapted to life in marine waters. They are recruited exclusively from two families, the Potamogetonaceae (7 genera with ca. 35 species) and the Hydrocharitaceae (3 genera with 12 species), and form together an interesting ecological group. Consequently, their taxonomy, morphology, flower biology, and geographic distribution have been much studied, especially by Ascherson (1868, 1889, 1906, 1907), Sauvageau (1890, 1891), Ostenfeld (1915, 1916, 1927), Setchell (1920, 1935), and Miki (1932, 1934). In spite of the work of these eminent investigators the taxonomy of several genera viz. Halodule, Posidonia, Zostera, and Phyllospadix is yet imperfectly known. One of the most serious gaps in our knowledge is no doubt the lack of ecological data; this greatly hampers the judgment of the biometric characters of the species with relation to their usefulness for taxonomical purpose. Less important is the fact that the generative parts of several species are partly or completely unknown. The taxonomy of the genus Halodule, which had been known for a long time under the name Diplanthera has been studied in the scope of the revision of the Potamogetonaceae for the Flora Malesiana. The development of the taxonomy of this genus has been seriously obstructed not only by the difficulties in the interpretation of the slight morphological differences between the species but also by the fact that nearly all investigators based their identifications on the works of Ascherson (1889,1906,1907). According to this author the genus Halodule contains two species: H. uninervis (Forsk.) Aschers. and H. wrightii Aschers. Although he mentioned differences in generative and vegetative characters, the difference in geographic distribution he regarded as more important. Specimens from the Indo-Pacific were referred to as H. uninervis and those from the Caribbean were called H. wrightii. The geographic character was stressed in particular by Ostenfeld: “On the whole it is not possible to distinguish the two species when sterile, except using their quite different geographical distribution as criterion.” (1902, p. 262). “Die zwei Arten der Gattung sind einander so ähnlich, dass es nicht sicher ist, ob sie als zwei Arten beibehalten werden konnen. Ganz wie beim Artenpaare Halophila Baillonis – H. decipiens sind die Verbreitungsareale eigentlich das beste Unterscheidungsmerkmal.” (1927, p. 47). No wonder that the specimens in the herbaria all seem to be identified according to the traditional geographic scheme, even when the morphological characters of the plants de not agree with the species descriptions.