A variety of methods has been proposed to study the toxicity of chemicals or polluted waters on ciliates as representative test organisms of the microfauna of aquatic ecosystems. These methods are based on morphological, ultrastructural, ethological, or metabolic criteria. The techniques proposed are mostly species dependent and so are their applications and restrictions. In view of a future standardization of protozoan toxicity tests a selection has been made by the authors on the basis of the applicability of the tests for routine analysis. The methods withheld are briefly described and comments are made on their advantages and limitations. The equipment involved, the operational complexity, and the accuracy of the results and their interpretation are listed in a synoptic table.