Acetylcholinesterase (AChE) inhibition is widely regarded as a good biomarker of exposure to organophosphorus pesticides (OP). However, less is known about the relationship between AChE inhibition and consequences for growth, reproduction and survival on organisms. Acute toxicity tests with fish have shown that high percentages of AChE inhibition are needed to cause detrimental effects, but not much is known about the consequences of chronic exposure to this group of chemicals for both AChE activity and higher levels of biological organisation. In this study, zebrafish (Danio rerio) were exposed to sublethal concentrations of the OP parathion for 250 days in a flow-through system. Besides AChE activity, a variety of other parameters were measured: whole-body protein and lactate content, consumption rate, survival, growth and reproduction. AChE inhibition was correlated with exposure concentration, but not with exposure time, and was significant above 0.9 microg/l after 144 days and above 4.3 microg/l after 250 days of exposure. Both parathion and the cosolvent dimethylsulfoxide (DMSO) significantly increased food consumption rate of the fish. Survival, growth, reproduction and lactate content were not affected, while protein concentrations showed only minor effects. These findings support the hypothesis that AChE is a very sensitive biomarker for exposure, but not accurately predict higher level adverse effects following long-term exposure to OPs.