Since the 1970s, water temperatures along the Atlantic seaboard of the United States have risen by an average of 0.5ºC in summer months and 2.2ºC in winter months. In response, the distribution and abundance of several nearshore species have changed dramatically, but no study has attempted to document whether estuarine populations have evolved greater thermal tolerance. Here, we re-examine the classic latitudinal cline at lactate dehydrogenase in the killifish Fundulus heteroclitus that was originally described by Dennis Powers and associates from samples collected between 1970-72. Laboratory and field evidence indicated that northern and southern isozymes at muscle lactate dehydrogenase (LDH) are locally adapted to cold and warm temperatures, respectively. Despite the potential for evolutionary response at this adaptive locus, we detected no significant shift of the LDH cline between the early 1970s and 2010. We conclude that microevolution of LDH-mediated thermal tolerance has not occurred, shifts in alleles are too incremental to be distinguished from random processes, or that F. heteroclitus uses phenotypic and genetic mechanisms besides LDH to respond to warmer waters.