Immature sheepshead minnows (Cyprinodon variegatus), inland silversides (Menidia beryllina), mosquitofish (Gambusia holbrooki), and grass shrimp (Palaemonetes pugio) were exposed to the mosquito adulticide, Permanone 31-66, in a series of static toxicity tests conducted in the laboratory. At 24 h, in order of decreasing susceptibility, LC50 values were 0.843, 4.07, 5.46, and 6.04 ppb for grass shrimp, inland silverside, sheepshead minnow, and mosquitofish, respectively. Forty-eight-hour LC50 values for grass shrimp, inland silverside, sheepshead minnow, and mosquitofish were 0.049, 2.86, 3.02, and 4.29 ppb, respectively.