The photosynthesis of Tidestromia oblongifolia (Amranthaceae) is remarkably well adapted to operate at the very high summer temperatures of the native habitat on the floor of Death Valley. The photosynthetic rate was very high and reached its daily maximum when the light intensity reached its noon maximum at the high leaf temperatures of 460° to 50°C which occurred at this time. At the intensity of noon sunlight the rate decreased markedly when the leaf temperature was experimentally reduced to below 44°C. The optimum rate occurred at 47°C. At this temperature the photosynthetic rate was essentially directly proportional to light intensity up to full sunlight.