Due to increased organophosphate (OP) pesticide applications, it has become necessary to evaluate their hazards and develop biological indicators of aquatic contamination. It has been hypothesized that suppression of ChE activity could be used as an indicator of contaminant stress and exposure. It also has been shown that selective toxicity of OPs could be due to variability among structure and properties of AChEs of different animals, as well as reactivation of the enzyme. One possible way to create highly selective chemicals is synthesis based on similarity in structure-activity relationships derived from biochemical data. The review summarizes published data on ChEs in aquatic organisms.