The present paper embodies the results of a study of 362 specimens of the genus Rivulus from Suriname and the other Guyanas. So far, 58 species names (morphological species or subspecies) have been proposed, by a great many authors; these names are listed on pages 52—53. Of the 58, topotypical specimens have been examined in 8 instances. In order to facilitate a future review of the genus, which is in great need of revision, short remarks are made on the morphology and ecology of a number of specimens, from various localities, belonging to distinct species.