Callitroga americana males were sterilized by irradiating young adult flies, but it was most efficient to irradiate pupae within 2 days of emergence with a dosage of 2500 roentgens of direct irradiation plus an additional 50% due to backscatter and secondary irradiation. A dosage of 5000 roentgens direct irradiation was required to sterilize females, and that treatment also rendered the females incapable of producing normal egg masses. When sterilized males were confined with normal males and normal males and normal females in laboratory cages, most of the normal females did not become fertilized if the sterilized males outnumbered the normal males by a ratio of 5 or 10 to 1. Adding sterilized females to the caged population along with sterilized maled did not affect the end result. Laboratory observations showed that male flies will mate as many as 11 times if virgin females are available. Most females mated only once although mated females remained attractive to males.