Toxicity and bioaccumulation in bluegill sunfish (Lepomis macrochirus) of 4-nonylphenol (NP), a common precursor and degradation intermediate of certain nonionic surfactants, were evaluated in a set of 18 aquatic mesocosms (littoral enclosures) in northeastern Minnesota. Nonylphenol was applied to enclosures every 48 h over a 20-d period (11 applications) at rates of 3, 30, 100, and 300 μg/L. Additional enclosures served as untreated controls. Maximum NP levels in the integrated water column 2 h after each application averaged 5 ± 4, 23 ± 11, 76 ± 21, and 243 ± 41 μg/L over the 11 applications at the 3-, 30-, 100-, and 300-μg/L treatments, respectively. Sixty-one percent of the NP dissipated from the water column within ˜39 h of each application. Estimated survival of stocked juvenile bluegills was significantly reduced at the 300-μg/L NP treatment. No significant effects were noted at the other NP treatments; however, the 100-μg/L enclosure with the highest mean NP concentration (93 ± 39 μg/L) within that treatment had four to nine times more dead bluegills and four to six times fewer bluegills captured at the end of the season than the other two 100-μg/L enclosures (64 ± 23 and 71 ± 32 μg/L) and the controls, suggesting increased mortality in this enclosure. Nonylphenol tissue concentrations in juvenile bluegills collected from enclosures treated with 3 and 30 μg/L NP ranged from 0.01 to 2.94 μg/g wet weight and showed a significant positive relationship with the average measured NP concentration in the water. The mean wet weight nonequilibrium NP bioaccumulation factor was 87 ± 124. There was no relationship between fish lipid content and NP tissue concentration.