Four natural populations and 4 laboratory colonies of Boophilus microplus (Canestrini) and a laboratory colony of Boophilus annulatus (Say) were subjected to electrophoretic analysis to determine levels of genetic similarity and genetic variability. Populations of B. microplus exhibited high genetic similarity (I = 0.984 +/- 0.012) indicating that all populations share a relatively undifferentiated gene pool. Host vagility is proposed as an important mechanism promoting gene flow in ectoparasites. Levels of genetic variability for B. microplus (h = 0.092 +/- 0.008; P = 0.330 +/- 0.050) were within levels reported for other arthropods. The average genetic identity of 0.716 +/- 0.013 between B. microplus and B. annulatus is of a level characteristic of sibling species. Diagnostic isozymes between laboratory colonies will facilitate identification of these morphologically similar species.