Uptake and persistence of thiotepa (the sulfur analog of tepa) by pupae ofAedes aegypti(L.) was studied using gas chromatography. An average of 39.3±1.74 ng of thiotepa equivalents/pupa was found in samples taken shortly after treatment in 0.5% thiotepa for 3 hours. Newly emerged adults contained an average of 2.59 ng/adult, and in 24-h-old adults, there was<0.5 ng/adult. Competitive mating tests indicated that the sterile males were not severely debilitated, since the average competitiveness index was 0.94, or just below the value for equal competition. The extremely low residues in 24-h-old adults coupled with the vigor of the sterile males means that thiotepa-sterilized males present a means for the autocidal suppression ofA. aegypti.