The sensitivity of butyrylcholinesterase (BChE) toward the inhibition by the organophosphorus insecticide (OP) parathion-ethyl was compared with that of other esterases in the fish three-spined stickleback. Earlier field and in vitro results had suggested the higher sensitivity to OPs of stickleback BChE when compared with acetylcholinesterase (AChE). In the present study, stickleback were exposed in vivo under environmentally realistic conditions using a short duration of exposure (1 h) and parathion concentrations of 0.01, 0.1, and 1.0 mug/L. Seventy and 80% of nominal concentrations, respectively, were measured in the 0.01 and 0.1 mug/L treatments. Following exposure, stickleback were maintained in clean water for 48 h (recovery), allowing the metabolic activation of parathion. After recovery, the activities of BChE (axial muscle, gills, liver), AChE (brain, axial muscle, gills), and carboxyleaterase (CaE, liver) were determined. Following exposure to 1 mug/L parathion, the BChE activity was significantly decreased in liver (similar to 60%) and axial muscle (similar to 30%), while its decrease in gills (similar to 30%) was not significant. No effects on BChE activity were observed with 0.1 and 0.01 mug/L parathion. The AChE and CaE activities remained unaffected with all parathion concentrations used. The results are discussed with respect to the potential application of stickleback BChE as a biomarker of OP exposure.