Federal regulations require pesticide producers to provide aquatic field testing data when suspected risks warrant such advanced studies. Aquatic mesocosm tests are intended to satisfy the field testing requirement and represent an ecosystem-level approach to toxicity testing and ecological risk evaluation. These tests will provide risk managers with descriptive information on the extent of adverse impacts likely to occur in aquatic systems that can be fully evaluated in risk–benefit analyses. A strategy that uses aquatic mesocosms (experimental ponds or in situ enclosures) to satisfy testing requirements is discussed and the essential elements of an acceptable test are outlined. Critical features of a mesocosm test design include mesocosm size, biological composition, duration of the test, and parameters to be measured.