Tepa uniformly labeled with C14 with a specific activity of 12 mc/mmole was used to study the metabolism and mechanism of sterilization in male house flies, Musca fomestica L. A wet-combustinon method was developed for C14-labeled compounds which gave quantitative recovery and reproducible results. The injection of C14-labeled tepa was adjusted to about 1 µg per male fly. Total recovery of radioactivity from treated flies and their excreta was almost quantitative; radioactivity in respiratory CO2 was only 0.75% of the injected dose during the first 12 hours. When male flies were allowed normal activities after an injection of about 1 µg of C14-labeled tepa, 50% of the dose was retained in the fly 5 hours after treatment. Radioactivity was present in treated flies as tepa or aziridinyl metabolites; radioactive metabolites in the excreta did not contain aziridinyl groups. Radiometric and colorimetric determinations indicated that treated flies retained about 9 and 5%, respectively, of the injected dose over prolonged periods. Radioactivity was transferred to female flies by copulation with treated males. Because this radioactivity was found within the female body and not on its surface, we concluded that the transfer was by copulation rather than by body contact. However, no detectable radioactivity was found in the sperm or the seminal fluid present in the spermathecae of inseminated females. An electron microscopic scrutiny of house fly sperm failed to reveal any structural changes brought about by the chemosterilant.