Rivulus marmoratus Poey, 1880 has previously been found only in south Florida, Cuba, the Bahamas, Barbuda, St. Martin and several islands off the Venezuelan coast (Curaçao, Bonaire and Gran Roque) (Hoedeman, 1958; Böhlke & Chaplin, 1968). In September of 1978, one of my students, Mr. Edgard W. Cabezas, brought me a specimen of this species that he had caught in the northern section of Maracaibo City, state of Zulia, Venezuela. We revisited the site, a poorly drained section of town near the western shore of the Straits of Lake Maracaibo, on 21.ix.1978 and 14.x. 1978, and collected an additional 19 specimens (15 of which are in my personal collection). Most of the fish were caught in shallow, brackish water (salinity 25°/00, temperature 3i°C) among algae growing on the flooded porch of an abandoned house, and among the debris, mangrove trees, and garbage dumped nearby. The water flowing through the dump originates from a series of open ditches that carry rainwater and sewage into the lake from the city. Thousands of Poecilia vandepolli Van Lidth de Jeude, 1887 occupy the same habitat. These specimens represent the first record for this species on the South American continent and the only record of Rivulus from the Maracaibo Basin. Their preference for habitats which are less than attractive to the average collector may in part explain why they have been overlooked until now. Description. — To facilitate future taxonomie comparisons of this population with the others, a summary of meristic data is presented below, and morphometric data are summarized in table 1. TABLE Ι