Despite widespread belief that selection molds thermoregulatory behaviors, direct evidence for fitness effects is extremely rare. We studied the effect of developmental temperature on embryo mortality in a viviparous snake. Seventy-four female Thamnophis elegans were maintained at one of nine constant temperatures during pregnancy (21–33 C). The duration of pregnancy was recorded for each female as well as the sex, snout–vent length, mass, and survival of 504 newborns. Embryo survival was highest at an intermediate temperature (26.6 C). The developmental temperature of maximum survivorship in T. elegans corresponds to the temperature that induces minimum developmental abnormality and the optimum temperature for whole organism performance. These three thermal optima together correspond with the average temperatures imposed on embryos by free-ranging pregnant females (26.6 C). This correspondence implies that thermal optima are coadapted to thermoregulation in T. elegans.