1. The effectiveness of mating of female Aedes aegypti that had previously been mated was compared to that of virgin females. Non-virgin females mated less readily than virgin females and copulation was of somewhat shorter duration. Genital union was firm, and insemination occurred in both virgin and non-virgin females, yet semen was generally not retained in the copulatory bursa of females that had previously been mated. This effect was most evident when one or two days had elapsed between matings. Multiple insemination, with utilization of sperm from both matings, however, was occasionally effective when less than 5 hours separated the first and second matings.

2. Factors derived from the accessory glands of the male and, to a lesser extent, the testes appeared to induce this loss of semen.

3. It was suggested that semen normally gels during mating and that loss of semen following second mating may result from a defect in this process.

4. These data indicate that female A. aegypti in nature may normally utilize sperm from but one male.