Incubation temperature is known to affect pigmentation in turtles and alligators, but the influence of other incubation parameters on pigmentation has not been reported previously. We examined the pigmentation of hatchling red-eared slider turtles, Trachemys scripta elegans, incubated under various temperatures and gaseous conditions in the course of three prior studies. Reduced levels of oxygen during embryogenesis did not affect pigmentation, although such levels had produced small significant changes in developmental time. Elevated levels of carbon dioxide during development had a greater influence on pigmentation than did incubation temperature without producing as great a lengthening of incubation time. Hence, the changes in pigmentation produced by temperature and carbon dioxide could not be simply a function of their effects on developmental time. The carbon dioxide and temperature produced parallel changes in plastral pattern. In contrast, carbon dioxide had marked effects on skin pigmentation that were not paralleled by those of temperature. In addition, there were some significant differences among clutches in the effects of incubation temperature and carbon dioxide on pigmentation. Some of the differences in pigmentation among incubation treatments for this species are similar in magnitude to those occurring naturally among related turtle species. The ability to alter various developmental sequences differentially by changes in incubation conditions will facilitate both developmental and comparative evolutionary studies.