Stocks of northern largemouth bassMicropterus salmoides salmoides, Florida largemouth bassM. s. floridanus, and both reciprocal F1hybrids were produced in Champaign, Illinois, with natural spawning techniques. The genetic composition of each of these stocks was confirmed electrophoretically. Overwinter survival of northern largemouth bass in central Illinois was significantly greater than that of Florida largemouth bass; the two reciprocal F1hybrids had intermediate survival. The absolute survival rate of all stocks decreased as the severity of the winter increased; however, the effects were most dramatic for the Florida subspecies. The northern largemouth bass also exhibited-greater second- and third-year growth than the Florida largemouth bass; again, the reciprocal F1hybrids were intermediate. Results of this study indicate the potential negative effects that may arise when Florida largemouth bass or hybrids between it and the northern subspecies are introduced into waters of the USA and Canada that are within or contiguous to the native range of the northern subspecies.