To investigate the occurrence of thermal adaptation in the zebra mussel, Dreissena polymorpha, inhabiting the lower reaches of the Mississippi River, we compared lethal heat-tolerance among three populations (mussels collected at Lake Pepin, MN; Alton, IL; and Baton Rouge, LA). We determined time-to-death at 32°C for 160 individuals per site, for mussels collected at a water temperature of 15°C and then maintained in the laboratory for about 8 wk under uniform conditions. Both shell length and condition index significantly affected survival time and were included as covariates in the analysis for interpopulation differences in heat-tolerance. Zebra mussels from our southernmost location had a higher heat-tolerance than those from the two northern locations. This difference in heat-tolerance among sites may indicate adaptation to local temperature regimes. In addition, in a comparison of heat tolerance within populations, we separated mussels into size classes (where larger mussels have been exposed to local conditions longer) and calculated an adjusted mean time-to-death (TTD). We found a different TTD/size relationship depending on sampling location. Minnesota mussels had decreasing heat tolerance as size increased, where Louisiana mussels had the opposite relationship. These patterns of heat-tolerance within populations indicate a selection pressure for increased heat-tolerance at Louisiana. However, even if the selection pressure is strong at the Louisiana site, it has not (at least not yet) resulted in an adaptation, as high heat-tolerance is not ubiquitous within this population. Zebra mussels may have insufficient genetic variation for heat-tolerance or gene flow may have been too strong for genetic adaptation to occur in the short amount of time that zebra mussels have occurred in the lower Mississippi River.