The only thing all pesticides have in common is that they are used to control pests. Otherwise, they come from almost every imaginable class of chemical. Everyone associated with pesticide use--farmers, Extension, EPA, state regulatory agencies, manufacturers, and environmentalists--needs information that will allow them to distinguish between pesticides that may be a problem as pollutants in certain situations, and those which may not. There are five basic properties that, when combined with information about site and use, provide much information about the potential of a pesticide to be a pollutant. These five properties are solubility in water, volatility, soil sorption tendency, persistence, and ionization potential. We have compiled the most complete collection of these properties available, using others' compilations but verifying values from the primary literature in many cases. A complete primary literature search was not done. For each parameter we suggest a "Selected Value" which we believe to be the best available, recognizing, however, that persistence and soil sorption are sensitive to specific site conditions. These Selected Values are being incorporated into pesticide environmental-impact risk assessment procedures by state and federal agencies, and are considered to be consensus values. However, there is a serious potential for misuse of these data, particularly the error of using small differences between active ingredients to make regulatory distinctions between them. The ability to relate these data to environmental impact is an essential need and is improving, but is currently at a primitive level.