1. The in vitro biotransformation of 14C-aldicarb was examined in liver, kidney, and gill microsomes from the rainbow trout (Oncorhynchus mykiss). 2. In all tissues the major metabolite was aldicarb sulphoxide. Addition of the cytochrome P-450 inhibitor, N-benzylimidazole, failed to alter significantly aldicarb sulphoxide levels, while co-incubation with the flavin-containing monooxygenase substrates, N,N-dimethylaniline or methimazole, caused significant decreases in sulphoxide formation in liver and gill microsomes. 3. Aldicarb sulphoxide formation was optimal at pH 8.0, and had Michaelis-Menten kinetics with an apparent Km of 46.7 microM and a Vmax of 0.216 nmol/min per mg. 4. Aldicarb sulphoxide formation was competitively inhibited by co-incubation with N,N-dimethylaniline in liver microsomes. These data indicate that flavin-containing monooxygenase plays an important role in the in vitro biotransformation of aldicarb in rainbow trout.