We conducted laboratory and field studies to test the hypothesis that volatile pheromones mediate mate location in the longhorned beetle Prionus californicus Motschulsky (Coleoptera: Cerambyicidae), an important pest of many agricultural and ornamental plants. Males were strongly attracted to live females and to carcasses of freshly killed females in olfactometer bioassays. Males also responded strongly to excised ovipositors of freshly killed females but not to their excised heads, thoraces, or abdomens. In field studies, males were strongly attracted to cages baited with live females. These findings demonstrate that female P. californicus produce a volatile pheromone from the ovipositor that attracts males over a distance, and they provide the first conclusive evidence of a volatile sex pheromone for a species of the primitive cerambycid subfamily Prioninae.