During the summer of 1969 the effect of the standard serial application of malathion used in mosquito control (85.7 g/hectare) on juvenile brown and white shrimp, Penaeus aztecus Ives and P. setiferus (Linnaeus) was examined under field conditions in a saline marsh near West Galveston Bay, Texas. Water samples contained a high concentration of malathion (2.0-3.2 ppb) immediately after application and a progressive reduction thereafter. Shrimp at treated stations exhibited mortalities ranging from 14 to 80%. Malathion concentrations in live shrimp taken from the test sites were consistently higher (0.5-3.2 ppm) than concentrations measured in dead shrimp removed during the same period (0.5-2.1 ppm). No deaths attributable to malathion were observed at the control stations and no detectable traces of malathion were observed in water or shrimp samples removed from the control stations during the experiments. It was concluded that malathion was the most likely contributor to brown and white shrimp mortalities observed at the treated station.