The report is a summary of Phase I and II of a study designed to provide necessary data and recommendations for development of estuarine acute and chronic atrazine criterion for the Chesapeake Bay. An updated summary of existing toxicity data showed that acceptable acute toxicity data were available for eight animal species in eight different families. These data will provide the necessary information required by the U.S. Environmental Protection Agency for the development of an acute estuarine criterion for atrazine. Data are needed from four different families (including one plant) for a chronic estuarine criterion. Available data can be used to fill the requirements for three of the families needed; however, one additional chronic test will be required.