The acute and sublethal effects of etofenprox, a nonester pyrethroid, was determined in narrow-clawed crayfish (Astacus leptodactylus Eschscholtz, 1823). Semistatic bioassay procedures were followed in both experiments, and the 24, 48, 72, and 96 h LC50 values (with 95% confidence limits) of technical etofenprox for crayfish were calculated as 0.68, 0.61, 0.45, and 0.41 mu g/L, respectively based on Finney's probit analysis. Two concentrations of etofenprox (0.04 and 0.1 mu g/L) were tested to determine sublethal effects due to 96 hours exposure. After exposure to sublethal etofenprox, hemolymph glucose, and lactate levels increased while total hemocyte counts and sodium levels decreased (p<0.05). Hemolymph calcium, potassium, magnesium, and chloride concentrations did not change significantly. Histological alterations were evident in the gills and hepatopancreas after exposure to sublethal etofenprox concentrations. Lamellar hyperplasia and lining in the afferent and efferent branchial vessels were recorded in gills; whilst tubule necrosis was obvious in hepatopancreas. Etofenprox was found to be very highly toxic to crayfish, a nontarget organism. Exposure to sublethal concentrations for 96 h affected circulating hemocytes and hemolymph stress parameters via histological response, to compansate for the adverse effects of etofenprox. (c) 2014 Wiley Periodicals, Inc. Environ Toxicol 30: 887-894, 2015.