The report provides a synopsis of field studies of Rivulus marmoratus from two population surveys of mangrove islands adjacent to the Belize barrier reef and observations made over fifteen years at several sites in South Florida. This small, cryptically colored killifish is the only known vertebrate selfing hermaphrodite. Florida populations consist nearly exclusively of hermaphrodites (> 99%), while the Belize populations contained a significant proportion (10-25%) of males. The combined observations demonstrate that this species is not 'rare' as previously thought, but elusive and highly adapted to microhabitats within mangrove forests. Standard ichthyological collecting techniques are ineffective in the habitat and have previously failed to reveal the strength of the association of R. marmoratus with the mangral ecosystem. (Copyright (c) 1990 by Verlog Dr. Frederick Pfeil, Munchen, FRG.)