We used a stepwise linear discriminant analysis to investigate the population structure of summer flounder, Paralichthys dentatus (Linnaeus). Analysis was based on 18 morphometric and meristic variables taken from 1,214 specimens collected in coastal waters between Montauk Point, New York and Cape Canaveral, Florida. Two populations were identified: one in the Middle Atlantic Bight, or between New York and Cape Hatteras, North Carolina; the other in the South Atlantic Bight, or between Cape Hatteras and Florida. Discriminant analysis coefficients, based on five morphometric variables taken from specimens collected at geographic extremes of the survey area, provide a mathematical means for classifying summer flounder into either the northern or southern population with an accuracy of 93%.