The prevalence of eastern mosquitofish (Gambusia holbrooki) infected with larvae of Eustrongylides ignotus declined during an 8-yr period from 54% in 1990 to 0% during the last year at a site near a sewage inflow into a watershed in north-central Florida, U.S.A. Treated human sewage had been discharged into this watershed for about 68 yr, but this practice was discontinued in the fifth year of our study. During the last 3 yr of the study, samples of mosquitofish, oligochaetes, water, and sediment were examined in detail from this site plus 3 other sites along the watershed. The density of aquatic oligochaetes was greater at the site near the historic sewage inflow than at other sites and did not change significantly during the later part of the study. Initially, dissolved oxygen (DO) in the water column was lower at the site of the historic sewage inflow than at other sites, whereas mean sediment, total nitrogen, total carbon, and total phosphorous were greater. Dissolved oxygen increased and the other parameters declined after sewage input was terminated and coincided with a decline in numbers of infected fish. Although other factors, such as human disturbance and changes in wading bird foraging patterns, may have contributed to variations in the prevalence of infected fish, we conclude that discharge of sewage effluent into wading bird foraging areas may result in increased prevalences of fish infected with eustrongylid larvae.