A critical first step for environmental risk assessment is problem formulation to identify environmental entities of concern and determinants of exposure that place these entities at risk. A conservative screening level approach was used to evaluate the potential risk to sensitive aquatic species from maize-expressed Cry1A(b) protein occurring in a representative agroecosystem. Estimated environmental concentrations for Cry1A(b) were compared to threshold concentrations of concern for putative sensitive aquatic organisms as estimated from species sensitivity distributions. The high-end risk expressed as the combined probability of short-term exposure and acute effects to a sensitive species indicated no concern in 99% of cases with limited opportunity for chronic effects due to the rapid decline of Cry1A(b) from the environment. Addressing uncertainties in the distribution of Cry1A(b) in soil, water, and sediment clarify the need for expanded ecotoxicity testing for aquatic effects.