Diiodomethane at a concentration of 10 microgram/ml increased the median life span of the fruit fly, Drosophila melanogaster, by 29.3%. Decreased flight activity was associated with increased longevity when the flies were reared and maintained on diiodomethane. Rearing only on diiodomethane increased life span by 13.6% and eliminated the decreased-flight-activity phenomenon. The effects of iodine, iodoform and hydriodic acid on longevity were also examined.