Dermacentor variabilis and Dermacentor andersoni males were each fed in the presence of o, 1, 2, 3, 6, 12 and 25 females and each group was analyzed for time of voluntary movement to seek females and rate of spermatogenesis. Males migrated earlier when females were present and were more prone to migrate with immature gonadal development in the presence of larger numbers of feeding females. However, spermatogenic rate appeared to be constant whether females were present or not. Since early detachment of D. andersoni males was stimulated by fewer females than the same response in D. variabilis, it is believed that D. andersoni males are more sensitive to the pheromone corroborating the finding of SoNENSHINE, et al. (1976) indicating greater sensitivity of D. andersoni than D. variabilis to the sex pheromone.