1. The initial slopes of the substrate-activity curves of several hydrolases were determined in the microsomal and cytosolic fractions of the liver of several fish recommended by OECD for the regulatory testing of chemicals. 2. Inter-species differences ranged within a factor of 7-17 for the esterases and reached a factor of 60 for the amidase. Guppy and carp appeared endowed with hydrolase activities which, overall, are much higher than zebra fish, trout and golden orfe. 3. The comparison with the rat liver microsomal hydrolases strongly suggests that fish are endowed with similar or higher levels of A-esterase and with much less B-esterase/amidase activities.