In the southern tip of the Everglades, Florida, a small population of the American crocodile, Crocodylus acutus builds nests in two quite different substrates, sand/shell and marl. Changes in temperature, soil water and gaseous composition were monitored in selected nests throughout incubation, and the gaseous resistance of the soils measured. Temperatures increased from approximately 30 C to a maximum of 34 C over incubation and no differences were found between the two nest types. The marl nests had a higher water content than the sand/ shell nests and had a significantly higher resistance to gaseous diffusion. In both nest types there was a decline in PO2 and an increase in PCO2 over incubation, with greater extremes reached in the marl nests. In sand/shell nests, eggs lost 15% of initial wet weight over incubation. It is suggested that the crocodile embryo adapts to the characteristics of the different substrates by matching its metabolic rate to the gaseous environment of the nest. Important similarities in bird and crocodilian egg development suggest that the birds have been highly conservative in this feature of their biology.