SUMMARYThe diatomAchnanthes exigua, isolated from Alhambra North Hot Spring, showed a maximum growth rate of 2 doublings per day at 40 C; the maximum and minimum temperatures at which growth occurred was 44 and 10 C, respectively. The optimal temperature for respiration was 40 C, whereas the optimum temperature for photosynthesis was 42 C. Respiration exhibited greater thermotolerance than either growth or photosynthesis. Light saturation occurring at comparatively low light intensities suggests that the unialgal diatom culture may represent a selected shade‐adapted organis