Ichthyological investigations in Suriname, carried out in connection with the "Brokopondo project", financed jointly by the Rijksmuseum van Natuurlijke Historie and the Stichting voor Wetenschappelijk Onderzoek in de Tropen, have yielded large collections of freshwater fishes. These collections will form the basis of a number of systematic papers and revisions, by several authors, of which this, dealing with a single genus, Heptapterus, of the family Pimelodidae, is one. For more particulars on the collections, etc., I refer to Boeseman (in press); a map and a description of collecting-localities will be published later. Unless otherwise stated, the material is in the Rijksmuseum van Natuurlijke Historie, and the numbers given, are the registered numbers in that collection. The X-ray photographs, from which the vertebrae were counted, were taken for me by Mr. J. Simons (Zoologisch Laboratorium, Leiden); the figures 2 to 4 were drawn by Mr. W. C. G. Gertenaar of this institute. Five species of the expanded genus Heptapterus as defined below are known from Suriname. Heptapterus Bleeker Heptapterus Bleeker, 1858, Visschen Ind. Arch. 1: 197 — type by monotypy, Pimelodus mustelinus Valenciennes. Acentronichthys Eigenmann & Eigenmann, 1889, Proc. Calif. Acad. Sci. (2) 2: 28 — type by original designation, Acentronichthys ieptos Eigenmann & Eigenmann. Chasmocranus Eigenmann, 1912, Mem. Carnegie Mus. 5: 131 (in key), 160 — type by original designation, Chasmocranus longior Eigenmann. Long, slender Pimelodidae lacking spines in D and P, and with the