Evidence is presented to show that the regular production of "unisexual" progenies in Sciara coprophila is not due to a differential mortality of zygotes but to selective fertilization. It involves 3 separate possibilities: (1) that of differential mortality occurring after the eggs are laid; (2) that of its being brought about by a selective retention of fertilized eggs by the ; and (3) that of its being due to degeneration of fertilized eggs within the body of the . Evidence on the first 2 points is clear cut, showing that no such mortality as would be required occurs after the eggs are laid, and likewise that eggs are not regularly retained by the . Evidence concerning the last feature is negative in nature, being based on the lack of any signs of degeneration, but it appears, neverthe-less, to be equally conclusive. It is concluded that a selective elimination or inactivation of sperms occurs, such that in [female] -producing [female][female] the Y-bearing sperms do not function and in cf-producing [female][female] the X-bearing sperms do not function (save for occasional exceptions in each case). There is no evidence at present to indicate whether this elimination or inactivation is effected by the egg or by something operating in the sperm receptacles or ducts of the [female] before the sperms reach the eggs. Additional evidence is given to support the premise that both types of sperms are present and that the results are not due to the d1.