Anthocoris antevolens White (Heteroptera: Anthocoridae) is a widespread predatory bug in North America commonly associated with deciduous trees and shrubs. Unpublished observations showed that there is considerable geographic variation in male genitalia in this species and that the variation may lead to reproductive isolation among geographically separated populations. We show that male bugs from two sympatric populations in the Yakima Valley, Washington, one occurring on oak (Quercus garryana Douglas) and the other collected from willow (Salix sp.), differed in size and shape of the phallus and clasper. Mating trials showed that males from the oak source successfully inseminated females from the oak source in 75% of pairings; insemination success for males from the willow source paired with females from the willow source was somewhat lower at 62%. In nonlike crosses (oak × willow, willow × oak), males failed to inseminate the female in 100% of pairings, despite vigorous mating attempts by the males. Copulation duration was independent of population source. However, males from the willow source initiated copulation attempts significantly sooner in the assay than males from the oak source, irrespective of female source. We interrupted copulating pairs by freezing them with liquid nitrogen and showed that males in nonlike crosses generally had failed to fully inflate the phallus in the female. Results support statements made elsewhere that A. antevolens is actually composed of an unknown number of reproductively isolated cryptic species.