Field-collected Anopheles quadrimaculatus and An. punctipennis adult mosquitoes from various types of overwintering hibernacula in southwestern Michigan and northern Indiana were examined for cold tolerance. Adult mosquitoes were collected on a monthly basis from October 1997 to April 1998. The mean supercooling point for adult for An. punctipennis was significantly lower than that for An. quadrimaculatus. The lower lethal temperatures for these species were -17 degrees C and -15 degrees C for An. punctipennis and An. quadrimaculatus, respectively.