The Technical Guidance Document for aquatic mesocosm tests to support pesticide registrations is a document primarily intended as guidance for Agency reviewers and scientists in the regulated industry to facilitate the development and completion of an acceptable aquatic mesocosm test. As a result of application for registration, pesticide producers are required by 40 CFR Part 158.145 to provide aquatic field testing. Aquatic mesocosm tests are intended to satisfy the field testing requirement. The document outlines the essential elements of an acceptable test. The Technical Guidance Document is the culmination of recommendations generated out of a workshop held at George Mason University, Fairfax, VA April 8-9, 1986. An aquatic mesocosm test represents an ecosystem-level approach to toxicity testing and ecological risk evaluation.