A colony of the yellow-fever mosquito, Aedes aegypti (L.) , which had been selected by exposing the larvae to 5-25 ppm of apholate for 43 generations, was 20 times as resistant as the regular colony to the sterilizing effect of larval treatments with apholate. Little cross resistance to tepa was evident, though the selected strain did show a 3- to 4-fold increase in resistance to metepa. At high con centrations all 3 compounds caused extensive pupal and adult mortality, although the apholate-resistant colony could withstand higher dosages of apholate (but not tepa or metepa) than regular colony without any detrimental effect on development. Adults of the selected colony showed only a 6- to 7-fold increase in resistance to the sterilizing effects of apholate when it was administered by ad libitum feeding in sugar solution.