The effects of apholate on diapausing and nondiapausing alfalfa weevil, Hypera postica (Gyllenhal), were studied. Different stages, sexes, and ages of the weevil were treated with aqueous-apholate solution. Adult topical application and larval and adult dipping methods were used in the experiment. Both showed the effectiveness of sterilization, and caused high mortality to the treated insects. Adult dipping was an acceptable method of treatment. The best age for treatment seemed to be reproductively mature virgin adults. Egg viability was similar to the check when only the females were treated with apholate. When only males or when both sexes were treated the egg viability was decreased depending on the apholate concentration used. The most effective apholate concentration for sterilizing male alfalfa weevils ranged from 0.5 to 1.0%. A gradual increase in egg viability occurred starting about the 3rd week after treatment. Dissection of the immature adults showed that gonadal development was delayed by apholate. The reproductive organs of apholate-treated mature adults were reduced in size. The treated males do not compete sexually on an equal basis with the normal males, although considerable mixing of sperm occurred during the mating of female weevils with the apholate-treated and -untreated males. / M.S.