Although sex pheromones have been identified in many species of ticks, little is known about the role of sex pheromones in the black-legged tick, Ixodes scapularis. Bioassays showed that unfed deer ticks mated readily prior to feeding. Unmated ticks also commenced mating within a few minutes after acquiring a vertebrate host, and most females were mated within 48 hours. Surprisingly, males also were attracted to and copulated with replete females, but failed to mate with partially fed females. Washing females with a lipid solvent eliminated the response. Males also rejected dead females. Adding extracts made from the lipid washings of females failed to restore the response. Males ignored females enclosed in cloth or perforated capsules, indicating that the sex pheromone is non-volatile. When dead females were treated with an ecdysteroid, numerous males mounted and probed the genital pore, but none actually copulated. Chemical analysis (HPLC) of the surface composition of female ticks showed a mixture of cholesteryl esters, confirmed by mass spectrometry, and one or more ecdysteroid-like compounds. However, the identity of the ecdysteroid-like compound(s) has (have) not been determined as of the time of this report.