1. During the larval life of Aedes aegypti (L.), the testes greatly increase in size and numbers of germinal cells, and the number of compartments (or spermato-cysts) increases about five times, to a maximum of 24, usually several days before pupation. Although the germ cells may begin the process of differentiation of spermatids into spermatozoa within the terminal cysts of the testes just before pupation occurs, fully differentiated spermatozoa were never observed in larvae. Generally one testis is smaller than the other in fourth stage larvae, and this difference tends to persist throughout life.2. Although a significant increase in the number of spermatocysts could not be detected during pupal life, as many as 29 cysts were found among the testes examined during this period. Spermatids may transform into fully differentiated spermatozoa within five hours. While the beginning of differentiation of the spermatids may be delayed for as long as 21 hours after pupation, differentiated sperm were always fou...