Abstract There is evidence for the existence of a previously undescribed sex pheromone (or pheromones) in the ticks Dermacentor variabilis and D. andersoni. In addition to 2,6-dichlorophenol, which attracts mate-seeking males, a pheromone released on the cuticle of the female genital area enables the sexually excited male to locate the gonopore. The compound (or compounds) appears to act as a contact pheromone; male copulation responses are greatly reduced when the female genital surface is washed with solvents, especially hexane. It is also a potent excitant; males will puncture or dislodge barriers placed over the gonopore to copulate. However, the response is eliminated if the genital area is washed (hexane or acetone) prior to sealing the gonopore, suggesting the reproductive system as the source of the pheromone. A species specific copulation-eliciting pheromone appears necessary to excite the male to form and implant its spermatophore in the vulva of a conspecific female. Males encountering trans-specific females probe their gonopores, but mating attempts are almost always aborted within 5–10 min. The copulation-eliciting pheromone may be the same, or similar, to that used to locate the gonopore. Physical differences in the shape of the female gonopore in the two species, although slight, may contribute to the male's ability to identify conspecific females. Using this pheromone-guided process of attraction and identification, females present in mixed species populations will almost always be distinguished and inseminated by conspecific mate-seeking males.