A symposium on the use of aquatic mesocosm studies in the ecological risk assessment of pesticides was held at the 11th annual meeting of the Society of Environmental Toxicology and Chemistry, in Arlington, VA in November of 1990. The objective of the symposium was to review the “state of the art” in the area of aquatic mesocosm testing, with special emphasis on the use of these studies in the pesticide regulatory arena. Confirmation of laboratory-based risk assessments via mesocosm studies is an area of active research, and this symposium served to bring together experts for discussion in areas such as study design, methodology, interpretation, and regulatory evaluation.