Epoxide hydrolase of microsomal membranes of the common dab (Limanda limanda) has been characterized using p-nitrostyrene oxide as substrate. Under the conditions of assay used, the turnover number with this substrate was higher than found for the more frequently used styrene oxide and steady state kinetics were observed. The enzyme had a KM of 0.12 mM and optima for pH and temperature between pH 8-10.2 and 50-60°C respectively. Enzyme activity was unaffected by low concentrations of ionic and non-ionic detergents but was inhibited by higher concentrations of Lubrol and Brij. The enzyme protein did not react with monospecific antibodies to rat or human microsomal epoxide hydrolase during Western blotting. Large inter-individual variation in enzyme activity was found but the enzyme does not appear to be expressed in a gender-specific way. Fish were administered a wide range of hydrocarbons which are known to alter the expression of cytochrome P450 1A but these had no effect other than benzothiophene which caused a small increase in enzyme activity.