Understanding the interface between spatial and thermal ecology is integral to understanding energy acquisition and the life histories of ectotherms. Snapping turtles (Chelydra serpentina) occupy a wide range of habitats that vary greatly in their thermal properties. We studied activity, movements, and thermoregulation of C. serpentina in a small, land-locked lake in central Michigan, USA, using radiotelemetry. Consistent with our a priori predictions, turtles were active within core areas along the lake's edge, showed both diurnal and nocturnal activity, and did not make extensive interwetland movements. Turtles left the lake only for nesting or to hibernate in Sphagnum peat or in the banks of a nearby stream. Home range and core area size estimates of C. serpentina were small compared with other previously studied populations, perhaps in part because of the small dimensions of our lake. Contrary to our prediction of a broad T_set (thermoregulatory set-point) range for a large-bodied, habitat generalist turtle, we found a comparatively low and narrow laboratory-determined T_set range (22°C–26°C). Turtle body temperatures (T_b) cycled between May and August and attained maximal values during the evening hours, a pattern that likely results from thermal inertia, the selection of aquatic thermal patches, or both, as T_e (operative temperatures) declined. Turtles most effectively maintained T_b within T_set during July and August when thermal conditions were most favorable. Throughout most of the active season the highly aquatic habits of C. serpentina apparently negated the effects of variations in daily weather conditions and their effects on incoming levels of solar radiation on variation in T_b. However, turtles maintained higher average T_b on sunny days compared with overcast days in April and May, the coolest months of the study. Diel T_b cycling ceased during September and October and average T_b declined despite favorable thermal conditions, at least during September, a pattern that could reflect a downward shift in the T_set range. Comparatively, C. serpentina is less effective at thermoregulating than is a small-bodied species at a similar latitude (Chrysemys picta marginata). Apparently, thermal inertia and lack of atmospheric basking proclivities influenced the thermoregulatory precision in our turtles.