Temperate-zone amphibians generally show an acclimation response of tolerance to high temperatures (CTMax). The Puerto Rican frog Eleutherodactylus portoricensis failed to show an acclimation response, but did show a negative correlation between elevation and CTMax. Subsequently, E. portoricensis, as originally understood, was found to consist of a widespread species (Eleutherodactylus coqui) and a species restricted to high elevations (E. portoricensis). CTMax was determined for these 2 species. Differences previously believed to be attributable to elevation are, in fact, interspecific differences. There were no differences in CTMax of E. coqui from different elevations, but E. portoricensis had significantly lower CTMax. E. coqui failed to show an acclimation response. The few species of tropical amphibians that have been investigated suggest that, unlike most temperate-zone species, some tropical species fail to show an acclimation response, and some fail to show a negative correlation between CTMax and elevation.