A comparison of the mean life-spans for populations of Drosophila melanogaster at 10 degrees , 20 degrees , 25 degrees , and 30 degrees C maintained on media prepared with distilled water and with 50 percent deuterium oxide shows that deuteration decreases longevity at all four temperatures. The magnitude of the difference between the mean survival times of populations maintained on deuterated and nondeuterated media is inversely related to temperature between 10 degrees and 30 degrees C.