Mosquitoes of 3 species (Aedes aegypti, Ae. albopictus, and Culex quinquefasciatus) were fed on human blood containing various concentrations of ivermectin. Three effects (death, decreased egg production, and reduced egg hatching) were observed in the insects, depending upon the concentration of ivermectin ingested. The LD50 of ivermectin in human blood for the 3 mosquito species was estimated to be 126, 208, and 698 ng/ml, respectively. Mosquitoes dying after ingestion of ivermectin developed signs of acute toxicity including paralysis, lethargy, incoordination, and difficulty in movement. Death usually occurred within 48-72 hr. With sublethal blood concentrations of the chemical, mosquitoes survived, but there was a marked reduction in both the number and viability of their eggs. This infertility was only temporary, however, as subsequent refeeding of the insects on uncontaminated blood resulted in the production of normal numbers of fertile eggs. Blood levels of ivermectin which made 50% of the eggs infertile in Ae. aegypti and Ae. albopictus were calculated 3.4 and 4.3 ng/ml, respectively. These latter concentrations of the chemical are within the range found in blood of humans and domestic animals receiving ivermectin for treatment of parasitic infections. This finding suggests that the widespread use of ivermectin in veterinary and human medicine may have an unrecognized effect on mosquito populations.