Two isolates (North Carolina and Louisiana) of Lagenidium giganteum were introduced into natural freshwater mosquito breeding sites in North Carolina in June 1982 and their establishment and persistence monitored through the remaining mosquito breeding season. The North Carolina isolate on agar media was introduced into a flooded woodland that had populations of Anopheles punctipennis, Culex restuans, Cx. territans, Aedes vexans, Psorophora ferox and Ps. columbiae. A liquid culture of the Louisiana isolate was added to a flooded depression that was a source of An. punctipennis, Cx. restuans, Cx. territans and Ae. vexans. The fungus became established at both sites, remaining at enzootic levels when larvae were scarce and recrudescing when larval populations increased after flooding. Infections during the observation period ranged from 0 to 100% among larvae collected from the sites and 20 to 100% among laboratory-reared larvae (Cx. quinquefasciatus) exposed in the sites for 24-48 hr. Mosquito larvae added to water samples drawn from the sites and held in the laboratory became infected with the fungus even when natural larval populations were very low or absent. Results indicate that L. giganteum recycled for the entire season despite periodic scarcity of hosts and short-term drought. When the water temperature dropped to below 18oC, infection of mosquito larvae by the fungus ceased.