Tests were conducted to determine if the Mermithid nematode, Romanomermis culicivorax Ross and Smith (=Reesimermis nielseni Tsai and Grundmann, auct., partim.), would effectively parasitize and develop in Culex quinquefasciatus Say larvae breeding in an abandoned sewage settling tank. Laboratory bioassays indicated that undiluted sewage water collected directly from the test sites on Sanibel Island, Lee County, Florida, had no measurable adverse affects on viability or infectivity of the preparasitic stage or on development of the parasitic stage. Field released of preparasites directly into 2 of the sewage settling compartments of the main system resulted in 37.3 and 53.7% parasitism of Cx. quinquefasciatus larvae at estimated parasite to host ratios of 3.4:1 and 4.6:1, respectively, and therefore indicated their potential use as biological control agents against mosquitoes which breed in some polluted water environments.