Three incidents with high mortality in Atlantic salmon after trichlorfon treatment against salmon lice are described. All 3 incidents occurred at water temperatures of 12 degrees C or higher. The mean brain acetylcholinesterase (AChE) activity of dead fish was less than 20% of normal activity, while survivors showed mean activities of 22-61% of normal levels. Dichlorvos residues in muscular and liver tissues ranged from nondetectable levels to 0.2 micrograms/g tissue. The strongest inhibition of brain AChE was found in association with the highest dichlorvos residues. Substantial AChE-inhibition was, however, also found in samples in which dichlorvos residues could not be detected. AChE-determination was found to be more reliable than residue analysis for the diagnosis of organophosphate poisoning in salmon.