Metabolic heating caused by physiological processes during the development of oviparous embryos can raise nest temperatures above those of the surrounding substrate and may be sufficient to increase embryonic growth rates, influence sex ratios of hatchlings with temperature-dependent sex determination, and increase hatching success in seasonal environments. In sea turtles with large clutch sizes, metabolic heating can raise nest temperatures by as much as 6°C. However, no studies have directly investigated metabolic heating in any species of freshwater turtle. We investigated whether metabolic heating occurs in nests of snapping turtles (Chelydra serpentina) from southeastern Michigan, United States. A temperature logger was placed in the center of 8 unaltered snapping turtle nests. A second temperature logger was placed at the same depth in the surrounding substrate 5 cm from the side of the nest chamber. Metabolic heating is more pronounced in nests with larger clutches, so we artificially increased the size of 2 additional nests using donor clutches of 11 and 21 eggs, respectively. Temperatures were recorded at 2-hr intervals until after the presumptive hatch date of all nests. We found that there was a significant increase both in mean nest temperature and accumulated heat units for natural and experimental treatment nests during the last third of incubation. Further, in nests with experimentally increased clutch sizes, mean nest temperature was significantly greater than substrate temperature throughout incubation, suggesting that large nests also exhibit a thermal inertia that results in positive heat balance throughout development, at least in the soils studied.