LC50 (24 hours) of permethrin to the tilapia Oreochromis aureus ranged between 6.23 and 6.54 micrograms/l (95% Cl 5.57 to 7.00) as measured in two static exposures at 24 deg C. No residues were detected in the muscle of O. aureus after an exposure of up to 4.8 micrograms/l. After exposure to 1.4 micrograms/l no residue was found in the gills. Permethrin concentration in the muscle reached 1,400 micrograms/g and in the gills 6,000 micrograms/g after exposure to concentrations of 12 to 14 micrograms/l. Residue concentrations in the gills were four times higher than those in the muscle. Permethrin concentration under static conditions decreased very quickly and long-term, low-level cases of fish poisoning in the field seem very unlikely.