Ontogenetic and interspecific differences in behavior and tolerance to low oxygen concentrations are reported for four species of Ambystoma larvae. 2. These differences are related to habitat and time of breeding. 3. Tolerance prior to hatching decreased with increasing embryonic states in all species. 4. Premetamorphic larvae were more tolerant than either small larvae or metamorphic individuals. 5. Activity was high upon initial exposure to low oxygen but decreased rapidly.