We investigated the effect of 4-nonylphenol (NP) on nematode communities in the sediment of freshwater microcosms. Seven treatments were dosed with various concentrations of NP over a period of six weeks by using a controlled-release method (NP1-NP7; maximum sediment concentrations: 0.29–3.37 mg/kg dry wt). Four microcosms were not exposed to NP and served as controls. Nematode communities were analyzed over a period of 15 weeks, including sampling dates before, within, and after the NP application. Communities were characterized in terms of total nematode abundance and species diversity (Shannon index and evenness), as well as composition of species, feeding types, and different life-history strategists (maturity index [MI]). Species composition was analyzed by using a multivariate method (principal response curves). Total nematode abundance and species diversity were not affected in any of the NP-treated microcosms. However, in the highest dosed treatment, NP-induced changes in the nematode communities occurred. Species and feeding types composition, as well as the MI, were affected in the postapplication period, with species composition being altered most clearly. In the highest dosed treatment, deposit-feeding species, classified as colonizers (Eumonhystera), increased in dominance, whereas epistrate feeders and chewers (Prodesmodora and Tobrilus) decreased in relative abundance compared to the control.