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The critical thermal maxima (CTmax) of two sympatric, diurnal, thermophilic harvester ants from the Mojave Desert, USA (Pogonomyrmex rugosus and P. californicus) were measured by ramping their temperature upwards at a rate of 0.25degreesC min(-1) during flow-through respirometry with optical activity detection. Rates Of CO2 emission (<(V)over dot>(CO2)) increased exponentially to plateau values that were twice as high in P. californicus as P. rugosus on a mass-specific basis. <(V)over dot>(CO2) then fell sharply, during which gross motor activity (measured optically) and spiracular control (measured from <(V)over dot>(CO2) variation) abruptly ceased, yielding two independent measures of CTmax. As determined by loss of muscular coordination, the CTmax of Pogonomyrmex rugosus was 51.57+/-0.38degreesC (mean +/- S.D., while that of Pogonomyrmex californicus was 51.74+/-0.25degreesC. As determined by loss of spiracular control, the CTmax of Pogonomyrmex rugosas was 51.59+/-0.35degreesC, while that of Pogonomyrmex californicus was 51.78+/-0.37degreesC. In each species a pronounced post-mortal peak of <(V)over dot>(CO2) was observed. The major ecological and behavioral differences of the two species are not reflected in their CTmax values, which do not differ significantly. ’Thermolimit respirometry’ allows CTmax to be estimated objectively with coefficients of variation (S.D./mean) <1%, lending confidence to comparisons between species or treatment groups.