A method has been developed that uses a simplified sample preparation procedure and atomic absorption or electrochemical detection of HPLC eluents for the determination of methylmercury in fish. Methylmercury is isolated from the blended sample by chloroform elution from a diatomaceous earth-hydrochloric acid column. The organomercury compound is then extracted into a small volume of 0.01 N sodium thiosulphate solution. An aliquot of this solution is injected on to a Zorbax ODS column and eluted with methanol-ammonium acetate solution (3 + 2) buffer, pH 5.5 containing mercapto-ethanol. Detection can be accomplished by atomic-absorption spectrophotometry with the aid of a specially designed apparatus for the generation of mercury vapour. Alternatively, a commercially available electrochemical detector equipped with a dropping-mercury electrode may be used.