Genistein and daidzein are two phytoestrogens, compounds that come from plants (especially legumes) and interact with the estrogen receptors present in humans, fish, and other animals. Although they are naturally produced in the environment, they can become concentrated in the effluent of facilities treating the waste of plant-processing industries. They are also detected in run-off from fields where plants such as red clover are found. At environmentally-relevant concentrations, genistein and daidzein have been shown to produce reproductive, behavioral, and immunosuppressive effects in fish. This work develops the chemical and microbiological parameters necessary to predict the concentrations of genistein and daidzein in a body of water receiving a discharge containing the phytoestrogens. The attenuation processes studied include direct and indirect photolysis, sorption to settling particles, and biodegradation. Biodegradation is shown to be an extremely efficient removal process. Work by the Aquatic Toxicology Lab at Saint Cloud State shows that estrogenicity is removed by biodegradation, but some component of the biodegradation product mixture produces an androgenic or anti-estrogenic effect. Therefore, more research is needed to fully understand the exposure of aquatic wildlife to phytoestrogens and their degradates.