Two cholinesterases (ChE) present in vertebrates, acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) are distributed in tissues in species-specific patterns. English sole (Pleuronectes vetulus, ES) and Hornyhead turbot (Pleuronichthys verticalis, HT) muscle ChE activities were measured in fish from a location near a municipal wastewater outfall and a far-field reference site off the coast of Southern California. ES muscle ChE correlated with total fish length (r=-0.3271) in fish from two sites. No significant differences were observed between gender or sampling sites. Substrate preference assays showed that both ChEs were found in muscle with BChE representing about 30% of the total ChE activity. Data resulting from the use of selective inhibitors suggested the presence of an atypical BChE. Higher sensitivity to pesticide inhibition was observed in ChEs from smaller fish. In HT muscle, ChE activities presented significant differences between males and females, but no correlation was found with total length. No significant differences were observed between sampling sites. Both ChEs were also present with BChE activity constituting 90% of the total ChE activity. Selective inhibitor results indicated the presence of an atypical ChE. Male ChE was more sensitive to pesticide inhibition. These data indicate that species, size, and gender dependent differences in ChE may contribute to differences in susceptibility to ChE-inhibiting toxicants encountered in the marine environment.