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, P. axtecus Ives and P. setiferus (Linnaeus) was examined under field conditions in a saline marsh near West Galveston Bay, Texas. Water samples contained a high conc 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 concs in live shrimp taken from the test sites were consistently higher (0.5-3.2 ppm) than concs 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 water shrimp mortalities observed at the treated station