Most terrestrial insect embryos support metabolism with oxygen from the environment by diffusion across the eggshell. Because metabolism is more temperature sensitive than diffusion, embryos should be relatively oxygen-limited at high temperatures. We tested whether survival, development time and metabolism of eggs of a moth, Manduca sexta, were sensitive to experimentally imposed variation in atmospheric oxygen availability (5-50 kPa; normoxia at sea level is 21 kPa) across a range of biologically realistic temperatures. Temperature-oxygen interactions were apparent in most experiments. Hypoxia affected survival more strongly at warmer temperatures. Metabolic rates, measured as rates of CO2 emission, were virtually insensitive to hypo- and hyperoxia at 22 degrees C but were strongly influenced at 37 degrees C. Radial profiles of P(O2) inside eggs, measured using an oxygen microelectrode, demonstrated that 3-day-old eggs had broad central volumes with P(O2) less than 2 kPa, and that higher temperature led to lower P(O2). These data indicate that at realistically high temperatures (32-37 degrees C) eggs of M. sexta were oxygen limited, even in normoxia. This result has important implications for insect population ecology and the evolution of eggshell structures, and it suggests a novel hypothesis about insect gigantism during Paleozoic hyperoxia.