Radioactive semen of male Anopheles quadrimaculatus Say and Aedes aegypti (L.), reared in media containing 0.1, 0.25, and 0.5 μ.c/ml of radiophosphorus per larva, was detected in the spermathecae of their mates after copulation. The radioautographic techniques employed for detection of semen from treated males can be utilized in field investigations of mosquito-mating behavior. Futhermore, male A. quadrimaculatus sterililed by tepa after semen tagging with radiophosphorus were competitivc with untreated males for mates among untreated females.