Seasonal changes in the critical thermal maxima (CT_max) of four species of aquatic insects were determined from February 2012 to February 2013 from a first-order stream in northern Lower Michigan. Three of these species: Stenonema femoratum (Ephemeroptera: Heptageniidae), Hydropsyche slossonae (Trichoptera: Hydropsychidae), and Dolophilodes distinctus (Trichoptera: Philopotamidae) exhibited seasonal changes in CT_max, increasing through the spring and summer and then decreasing into the subsequent fall and winter. CT_max of these species correlated strongly with both the seasonal ambient stream temperature and with a series of different laboratory acclimation temperatures, suggesting that organisms adapt to laboratory acclimation in a similar manner as they adapt to seasonal changes. In contrast, the CT_max of Parapsyche apicalis (Trichoptera: Arctopsychidae) remained constant regardless of ambient or acclimation temperature. All species exhibited greater thermal sensitivity relative to ambient temperature during the summer than the winter. Our study indicates that thermal tolerance patterns can be different among species in the same environment. It also provides the first winter and year-round thermal tolerance data for aquatic insects.