Accumulation, depletion and metabolism of 14C-labeled diazinon (100 mg/L) an organo phosphoric insecticide and acaricide was studied in a perch. Bioaccumulation factors of diazinon were found to be low. Depletion of diazinon and/or its metabolites in fresh water was fast. The substance was readily metabolized mainly by cleavage of the phosphoric acid moiety of the molecule. Highest concentrations of diazinon and/or its metabolites were found in the vertebral cord, lowest concentrations in the muscle. 14C-radiolabeled compound was synthesized in our institute. The synthesis is described.