Oxygen stress can slow development, induce hatching, and kill eggs. Terrestrial anamniote embryos face a potential conflict between oxygen uptake and water loss. We measured oxygen levels within eggs to characterize the respiratory environment for embryos of the red‐eyed treefrog, Agalychnis callidryas, a Neotropical frog with arboreal egg masses and plastic hatching timing. Perivitelline oxygen partial pressure (Po₂) was extremely variable both within and among eggs. Po₂ increased with air‐exposed surface of the egg and declined over the developmental period before hatching competence. Through the plastic hatching period, however, average Po₂ was stable despite continued rapid development. Development was synchronous across a wide range of perivitelline Po₂ (0.5–16.5 kPa), and hatching‐competent embryos tolerated Po₂ as low as 0.5 kPa without hatching. The variation in Po₂ measured over short periods of time within individual eggs was as great as that measured across development or surface exposure, including sharp transients associated with embryo movements. There was also a strong gradient of Po₂ across the egg from superficial to deep positions. Ciliary circulation of fluid within the egg is clearly insufficient to keep it mixed. Embryos may maintain development under hypoxic conditions by strategic positioning of respiratory surfaces, particularly external gills, to exploit the patchy distribution of oxygen within their eggs.