Montane neotenic Ambystoma tigrinum had lower critical thermal maxima (CTM) than desert neotenic A. tigrinum. In all other life history stages studied there were no differences in CTM between montane and desert populations. Comparison of life history stages in a desert population showed that larvae and neotenic adults had higher CTM than transformed animals or animals in the process of transforming. The CTM differences on life history stages reflects the potential thermal stresses to which these stages may be subject.