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Oxygen consumption is oftentimes used as a proxy for metabolic rate. However, pupfish acclimated to ecologically relevant temperatures may employ extended periods of anaerobism despite the availability of oxygen-a process we called paradoxical anaerobism. In this study, we evaluated data from pupfish exhibiting stable oxygen consumption. Routine oxygen consumption () of a refuge population derived from Cyprinodon spp. acclimated to 28 and 33 A degrees C was evaluated at the ecologically relevant assay temperatures of between 25 and 38 A degrees C. Different interpretations of the data are available depending on normalization. For instance, of smaller fish, measured per fish, was remarkably stable over a wide range of assay temperatures and was not different between acclimation groups. However, when measured on a mass-specific basis, in these same smaller fish increases more predictably as temperature increased. of refuge fish and the closely related pupfish, C. nevadensis mionectes, measured near their respective acclimation temperatures, were essentially identical. However, of 28 A degrees C acclimated fish of both species, when measured at 34 A degrees C, was greater than that of the 33 A degrees C acclimated fish measured at 28 A degrees C. We suggest that this observed ’efficiency’ may result from significant anaerobic metabolism use. Experiments investigating factorial aerobic scope (/) yielded values less than 1 in 21-36% of the 33 A degrees C acclimated fish. These values indicate a substantial contribution of anaerobic metabolism to energy utilization by these fish. However, muscle lactate levels are not elevated in exercising fish-a result that is consistent with paradoxical anaerobism use.