In many species of reptiles, sex is determined by the incubation temperature of the egg. In the red-eared slider turtle (Trachemys scripta), warm incubation temperatures produce females, cool ones produce males, and a narrow range of intermediate temperatures produces both sexes. The mechanism of sex determination has not been established. Some investigators have postulated that both total incubation time and developmental rate during the first third of development are better predictors of sex than is incubation temperature. Here we consider whether various oxygen concentrations might influence sex determination by altering total incubation time. We incubated T. Scripta eggs at various oxygen concentrations, and found that while total incubation times were significantly influenced by oxygen, sex ratios were not. Very low oxygen concentrations inhibited survival to hatching, In addition, total incubation time and the number of days to internalize the yolk after hatching were significantly lengthened by lowered oxygen concentrations. There were also strong effects of clutch on sex determination and physiology, but these effects were apparently not mediated by an effect on total incubation time.