Thirteen New World populations of Megachile rotundata and M. apicalis and two Old World populations of M. rotundata were analyzed at 23 enzyme loci using cellulose acetate electrophoresis. Conventional and G- or C-banded karyotypes were also examined in these populations. The principal findings are (1) No differences were found in the level of heterozygosity between haplodiploid Megachile and that of other diploid insects. This is the first record showing the mean heterozygosity in the hymenopteran genus to be consistent with the mean of most other diploid insects. (2) There was no difference in the level of heterozygosity nor the percent polymorphic loci between males and females, and heterozygotes was discovered in males of the two megachilid species studied. (3) The heterozygosity levels among New World populations of M. rotundata and M. apicalis were similar, but both were more than twice that of Old World rotundata analyzed. This is consistent with the expectations of the founder-event models of both Carson and Templeton. (4) FST, the amount of genetic differentiation among populations, was greater among New World populations of apicalis sampled than among those of rotundata. The extensive commercialization in rotundata may have tempered interpopulation differences which might had occurred had the species been undisturbed. (5) Wrights "isolation by distance" model is inapplicable to either of the two Megachile species. In rotundata, the high gene flow level suggests that human commercial action may effectively disrupt any opportunity for the development of locally adapted populations through selection. In apicalis, little gene flow is present among the populations which is expected by both Carson and Templeton models. The absence of any clinal pattern in apicalis probably better reflects Templeton's transilience model. We believe that the random alteration of major alleles, implicit in transilience, more readily accounts for the distinct local populations we found in apicalis. (6) Chromosomal data suggest that both pericentric inversions and deletions have been involved in karyotype evolution of the two species and a primitive karyotype is proposed based on its ubiquitousness in all populations of both species. / Graduation date: 1993