Karyotypes of nine species of Venezuelan annual killifishes were compared. Karyotypic differences were found between species and between some genera. A chromosomal sexual dimorphism was found in <u>Pterolebias hoignei</u> (2N = 46). Males of this species possess a single large "Y" chromosome. No chromosomal divergence was detected among conspecific populations from different localities. <u>Pterolebias hoignei</u> and <u>Pterolebias zonatus</u> were found to differ both in diploid number (46 and 42 respectively) and in metacentric chromosome number (6 and 12 respectively). All <u>Rachovia</u>species were found to have a diploid number of 44. <u>Rachovia maculipinnis</u>and <u>R. brevis</u> were found to differ from one another in metacentric chromosome number (20 and 12 respectively). <u>Rachovia hummelincki</u> and <u>R. pyropunctata</u> both possess metacentric chromosome counts of 10. No karyotypic differences were found between <u>Austrofundulus transilis</u> and <u>Austrofundulus limnaeus</u> (2N = 44, metacentrics = 12). Several populations of the latter although divergent in male color patterns, did not differ karyotypically. All species examined, except <u>Rivulus stellifer</u>, departed from the presumptive ancestral teleost diploid number of 48. The degree of chromosomal variation appears not to be as high among the New World as among the Old World annuals. This difference among lineages with similar life histories and reproductive strategies suggests that stochastic events have not played a major role in mediating chromosomal divergence in annual fishes. / Master of Science