The effects on brown trout,Salmo truttaLinn., and bottom fauna of the Silver Stream, Otago, New Zealand following an accidental discharge of 55 litres of a dieldrin spray in September 1963 are described. The 24 h LC50 for dieldrin was found to be 0.016 ppm. Residue levels of dieldrin in skeletal muscles, liver, and brain were measured, using a gas‐liquid chromatograph, and minimum lethal levels of dieldrin found in these tissues were 1.0–2.0 ppm.