A description is given of a mating plug that can be observed in recently fertilized females of Anopheles gambiae. This plug is normally absorbed in the course of the 36 hours following fertilization. Its presence in wild-caught mosquitos can be used for the recognition of nulliparous females. It is shown that the proportion of plug-positive and of unfertilized females in outside resting populations of A. gambiae was approximately doubled following the application of a residual insecticide to houses. The technique of dissecting and examining the mosquitos and a useful way of recording the results are described.