The jelly around amphibian eggs presents a formidable barrier to oxygen diffusion. Therefore, egg capsules must be thin enough, and the dimensions of globular egg masses small enough, to avoid oxygen limitation leading to developmental retardation or death. The eggs of the Australian moss frog, Bryobatrachus nimbus, have the thickest jelly capsule known for any anuran amphibian. Laboratory measurements of respirometric variables predict that single prehatching embryos should be normoxic between 5degrees and 20degreesC, with Po-2 in maintained above critical levels (10.2-17.0 kPa). However, numerical models of embryos amid larger egg masses (13-20 eggs) predict hypoxia at temperatures above 5degreesC. Contrary to model predictions, however, B. nimbus embryos rarely experience hypoxia in natural nests, because embryos occur in one or two layers and the moss substrate permits aeration of the lower surface while photosynthesis probably supplies oxygen directly. After hatching, larvae move to oxygen-rich regions of the jelly mass and disperse more widely within the mass as temperatures increase. Although nest characteristics relieve diffusive constraints, small clutch sizes, low rates of embryonic and larval respiration, and the cool climate occupied by B. nimbus are the main characteristics that prevent hypoxia.