The present paper is chiefly based on the Rivulid fishes collected by Dr. P. Wagenaar Hummelinck in the Antilles during the years 1930, 1936, 1937, and 1955, and in addition on some specimens collected by various other investigators at earlier dates. Some of the specimens, in particular those belonging to Rivulus marmoratus have been recorded before by Wagenaar Hummelinck (1933, 1940), Sanders (1936), and de Beaufort (1940). With the aid of Hummelinck’s field notes and water analyses, some further information could be given on the habitat and salt tolerance of the island Rivulids, cf. table 3. I wish to express my sincere thanks to Dr. K. H. Voous, Curator of Birds of the Zoological Museum of Amsterdam, and Professor of Zoogeography at the Free University of Amsterdam, for his aid and advice; to Dr. Ethelwynn Trewavas, Curator of Fishes of the British Museum (Natural History), London, for her kind cooperation in taking counts and measurements on ten of Boulenger’s types of Rivulus harti, and for sending me in exchange four topotypical specimens of Rivulus harti; to Dr. Leonard P. Schultz, Curator of Fishes of the Smithsonian Institution, Washington, for sending me additional material from Margarita; to Dr. M. Boeseman, Curator of Fishes of the State Museum of Natural History, Leiden, who lent me two specimens of Rivulus marmoratus from Curaçao; and finally to Dr. P. Wagenaar Hummelinck, Zoological Laboratory, Utrecht, for putting his Rivulid material at my disposal, and for his kindness and friendly suggestions.