We tested the hypothesis that larger egg and hatchling sizes in the snapping turtle confer posthatching advantages, by comparing the growth rates and survival of hatchlings from different thermal and hydric environments during embryonic development. We also tested for sex-specific differences in survival and growth, two indices of hatchling fitness, as predicted by the Charnov & Bull (1977) model of environmental sex determination (ESD). In addition, we examined interclutch and interpopulation variability in the responses of embryos and hatchlings to incubation conditions. Eggs of 21 clutches from four Ontario populations were incubated at 22·0, 25·5 or 29·3°C in a dry (–350 kPa) or wet (–150 kPa) vermiculite substrate, and hatchling survival and growth in the laboratory were monitored for 11 months. Initial egg mass and substrate moisture were positively correlated with mass at hatching, but did not affect hatching success, posthatching growth or posthatching survival. Initially heavier hatchlings were more likely to survive, but did not grow more quickly. The effects of incubation environment and hatchling mass were not consistent among clutches or populations. Intermediate incubation temperatures produced mostly males, with higher embryo and hatchling survival, as well as posthatching growth, than the mostly females from the extreme temperatures. These results support the Charnov—Bull model. Embryo and hatchling mortality and deformities were higher in clutches from two populations which are contaminated with organochlorine toxins. Interclutch differences were significant for all variables, even when initial egg or hatchling sizes were considered. Hatchlings from the population near the northern limit of the species' range grew the slowest. These results indicate that hatchling size cannot be used as an index of hatchling quality or posthatching success, unless interclutch and interpopulation variation are taken into account.