The hermaphroditic fish, Rivulus marmoratus, is the only vertebrate known to reproduce by internal self-fertilization; this process results in populations of homozygous clones. Most natural populations consist entirely of hermaphrodites, but phenotypically distinct, fertile males occur at frequencies up to 24% on some islands off the coast of Belize. The presence of large numbers of males in natural populations prompted this study to determine if males are involved in the mating system. The occurrence of cross-fertilization between males and hermaphrodites was determined by surveying progeny of field-caught hermaphrodites for non-segregation or segregation of DNA fingerprint markers as an indication of the homozygosity or heterozygosity of the parent. DNA fingerprinting revealed no segregation of markers among the offspring in 12 of 12 Florida and Brazil laboratory lines and in 5 of 30 Belize Cay broods. These data indicate that the hermaphrodite parents were homozygous; thus, no detectable outcrossing has occurred in these populations. However, DNA fingerprinting revealed segregation of markers among the offspring in 25 of 30 Belize Cay broods. Twenty-four of these broods were from the island of Twin Cays. An average of 30% of the parental bands were segregating among the offspring; values ranged from 0.09 to 0.50. Offspring were, on average, 8% dissimilar to one another; values ranged from 2.08% to 15.09%. These data suggest that the 25 hermaphrodite parents were heterozygous; thus, males are involved in the mating system in some Belize Cay populations. These data are the first evidence of outcrossing in this species. / Master of Science