Effect of microsporidiosis on the fecundity of hosts A. c. caspius was studied. The calculation of eggs in hatches has shown that females developing from infected eggs lay 2--3 times lesser eggs than non-infected ones. Besides, the number of females laying eggs decreases considerably. Despite the females of mosquitoes suck blood before each egglaying the disease, which proceeds at their larval stage, affects greatly their fecundity. Additional feeding of females cannot compensate completely the disturbances caused by microsporidiosis.