Most studies that have investigated the influence of hydric conditions during incubation on the size and quality of hatchling reptiles have used either vermiculite or sand as the incubation medium, and the applicability of data derived from eggs using one media type or the other to natural populations has had considerable debate. However, few studies have used soil from actual nesting areas as the incubation medium. In this study, I tested the influence of variation in substrate water content during incubation on the size of hatchling Snapping Turtles from southeastern Michigan using soil collected from actual Snapping Turtle nests. Eggs from six clutches were fully buried in sterilized soil that had been dried and then reconstituted to six different levels of hydration (3, 5, 7, 9, 11, and 13% gravimetric water content). Eggs in the driest soil (3%) had a slight net loss of mass over the course of incubation, whereas eggs in the wetter soils gained mass to varying degrees correlating positively with soil water content. Hatchling mass was significantly lower in the two driest soils than in the wetter soils. Carapace length was significantly shorter for hatchlings from the driest soil (3%) than for those from soils with 7% and 9% water contents, and significantly shorter for hatchlings from the wettest soil (13%) than for those from the 7, 9, and 11% water contents. Soil water contents measured adjacent to a single nest site over three consecutive field seasons ranged from 1.1 to 8.2%, with median water contents ≤ 5% in all three seasons. These findings support the hypothesis that normal variation in soil hydration can influence the size of hatchlings emerging from natural nests. The degree to which hatchling size varies with normal variation in soil water content, however, may have only minor impact on hatchling survivorship in light of other factors that could potentially influence hatchling survival in this species.