When females reared in the laboratory on artificial diet were gamma sterilized (LS) they became more receptive to the second mating than did normal ones (L), However sterilization reduced male effectiveness in second mating. Wild (W) females were found to mate more often when caged with LS males than when caged with W males. In mixed populations of W and LS flies, the latter tended to mate with each other at the first mating, but successive matings were more numerous between W females and LS males. After the first mating, ca. 0.21 matings per ♀-day were recorded between W females and LS males, as compared with 0.01 to 0.05 in each of the other three possible mating combinations during a 10-day period. It appears that, under laboratory conditions, W females were more receptive than LS females and LS males were more effective than W males to repeated mating.During first mating, W flies were always found to mate primarily in the last 2 h before scotophase, whereas L or LS flies mated in similar numbers during the 4 h before scotophase. After the first mating, mating activity in all fly types concentrated in the last 2 h before scotophase.