Virgin females of both Boophilus decoloratus and Boophilus microplus, when mated with males of the other species, subsequently produced sterile eggs. Counts of spermiophore capsules in female seminal receptacles showed that the males of both species will mate with the females of both species and that B. microplus males show a slightly greater, but statistically insignificant, mating capacity than B. decoloratus males. South African B. microplus females, when mated with an Australian strain of B. microplus males, produced a 62% yield of viable hybrid progeny while the reciprocal cross produced only a 1, 82% hatch of non-viable larvae. The hybrids were sterile when interbred and no hatch resulted when the Fl males were backcrossed with parent females. The reciprocal backcross of hybrid Fl females to parent males resulted in a low percentage hatch of non-viable larvae.