A mermithid nematode was isolated from all active stages of snow-pool Aedes spp. in northeastern New York. Invading nematodes initially entered the larval cephalic nervous tissue where limited development occurred, but subsequently migrated to the abdominal hemocoel of pupae and adults, where the rate of development increased sharply. Migration to the brain was a prerequisite for development in Ae. stimulons; nematodes not entering the brain elicited a host defense response. Most mermithids (64\%) entering this host were encapsulated and killed before reaching the brain, but field infection rates were still high, with more than 60\% of last-instar larvae successfully infected. Mermithids attacking Ae. intrudens and Ae. provocans seldom elicited an encapsulation response but nevertheless failed to complete parasitic development. Normal parasitic development with infrequent encapsulation was noted in Ae. excrucians and Ae. fitchii. Postparasites invariably emerged from adult stages, causing host death. Postparasites reared in the laboratory at 25 °C molted, mated, and began oviposition within 1 month of emergence. Resulting preparasites initiated infections in four laboratory-reared species of mosquitoes but seldom completed parasitic development. The biological control potential of this mermithid is discussed.