This guide was developed by the United States Environmental Protection Agency (EPA) as a taxonomic reference for the identification of diatoms. The taxonomic information included consists of a generic key to the common genera of diatoms, a section illustrating 164 diatom species representing 43 common genera found at the Water Pollution Surveillance System (WPSS) stations, and an alphabetical list of the genera included in this publication. Related information includes collection and slide preparation procedures, a description of the structure of the diatom cell wall, and a glossary of terms used in diatom taxonomy. Also included is a map of the WPSS station locations in the United States and charts indicating the four most abundant diatom species at these WPSS stations. No information is given on the ecology of diatoms. (MR)