The immune status of YOY winter flounder was evaluated in fish collected from six areas around Long Island, including Jamaica Bay, Little Neck Bay, Manhasset Bay, Oyster Bay, Port Jefferson, and Shinnecock Bay; sites geographically distributed from west to east, representing a large gradient in urbanization, and thus likely contaminant inputs from the water shed. Fish were collected from June to October, and expression of pleurocidin, a gene coding for an antimicrobial peptide found in skin mucus, as well as antimicrobial activity of skin mucus assessed in individual fish from each site. Cytochrome P4501A (CYP1A) expression was also measured as an indicator of contaminant exposure in these fish. Gene expression was quantified using qRT-PCR, and antimicrobial activity was assessed using a growth assay with Vibrio anguillarum and V. parahaemolyticus, two common marine bacteria known to affect winter flounder. Pleurocidin expression was ten times higher in fin tissue than in liver tissue, was highly variable between individual fish, and demonstrated no clear site specific differences associated with degree of urbanization of the watershed. Expression seemed to be related in part to fish size: a positive correlation between expression and total length of fish was observed, and fish in the largest size class (>125 mm TL) demonstrating significantly elevated expression as compared to fish in the smaller size class levels indicating that immune competency increases with age. Antimicrobial activity was also highly variable, showing no large site specific differences, and no significant correlation to pleurocidin expression. No differences in CYP1A expression were observed. These data suggest that exposure to aromatic hydrocarbon contaminants is fairly wide-spread throughout the study area and that any difference in pleurocidin expression in YOY winter flounder observed are due to other factors. The lack of correlation between pleurocidin expression and antimicrobial activity indicates that other antimicrobial peptides may be involved or that other factors are influencing antimicrobial activity. This is to my knowledge the first reports quantitatively evaluating pleurocidin expression in YOY winter flounder from an urban area. Further work is needed to characterize factors controlling pleurocidin expression, as well as other indicators of immune response in young fish.