Although behavioral endpoints are used in hazard assessment, establishment of water quality criteria and assessment of a contaminant's hazard to aquatic life rely primarily on standard acute and chronic toxicity tests. Sublethal effects of pollutants should, however, be of major concern because more organisms experience sublethal rather than acutely or chronically lethal exposures of contaminants. The avoidance-preference approach to behavioral bioassays is very useful in screening pollutants for which the mechanisms of perception or response are largely unknown. The underlying philosophy of these studies is that an animal which perceives a chemical can be “attracted” or “repulsed” by it. No response is frequently assumed to indicate lack of perception. All three responses have broad ecological implications. We discuss the conditions required for performing avoidance-preference bioassays, as well as their sensitivities, advantages, and limitations. In this regard, a comparative approach is used in examining the results of avoidance-preference bioassays with zebrafish in two different apparatuses. Finally, we compare the results of avoidance-preference studies with other measures of the behavioral toxicity of lead to tadpoles.