A mathematical model is presented for the temporal decline in total infectious units caused by simultaneous first-order inactivation and Brownian coagulation of viruses in an aqueous environment. On the basis of published physicochemical and biological constants for poliovirus, human immunodeficiency virus, and indigenous marine and freshwater bacteriophage, the model predicts that virion-virion coagulation is negligible in most aquatic systems. This analysis provides a framework for investigating the effect of coagulation and inactivation on viral infectivity and for developing more sophisticated models of virus survival outside the host cell.