Documentation is given for PEST, a dynamic simulation model for evaluating the fate of toxic organic materials (TOM) in freshwater environments. PEST represents the time-varying concentration (in ppm) of a given TOM in each of as many as 16 carrier compartments; it also computes the percent distribution and half life of the TOM in each of the carriers. Possible carriers include phytoplankton, macrophytes, zooplankton, waterbugs, zoobenthos, fish, particulate organic matter, floating organic matter, clay, and water (with TOM in the dissolved phase). PEST simulates TOM degradation by hydrolysis, oxidation, photolysis, microbial metabolism, and biotransformation by higher organisms; it simulates TOM transfer by solution, volatilization, sorption, absorption onto gills, consumption, excretion, defecation, biodeposition, mortality, and throughflow. These are subject to time-varying environmental factors such as pH, temperature, dissolved oxygen, wind, solar radiation, and biomass and condition of organisms. The model has been verified with process-level laboratory data and with ecosystem-level site data. The site data for fish ponds in Missouri and Israel and a reservoir in Iowa constitute prototype data sets that can be used to evaluate other compounds.