The closely relate cattle ticks, Boophilus microplus (Canestrini) and B. annulatus (Say), were tested for possible assortative mating under conditions designed to mimic those in the field. Patterns in the numbers of the four possible types of matings were generally indicative of a preference for conspecific mates in both species. There were significantly more conspecific and fewer interspecific matings than would be predicted from the observed frequencies of males and females among the two species had they mated at random.