Earlier work has shown that Drosophila is sterilized at 29-33[degree]C, but that these flies later recover fertility if returned to the optimum temperature of 24[degree]. Experiments here recorded bring out the following conclusions: High temperature has a differential effect such that males may be rendered completely sterile at a point at which females are still completely fertile. Most sterilized males recover fertility after being returned to the optimum temperature if the exposure does not exceed 10 days. The high temperature seems to cause loss of motility of the sperm, with progressive degeneration, which may include earlier germinal cells.