Descriptions of the diatom species which are most frequently encountered at WPSS stations have been assembled in this illustrated guide which was prepared to serve as a bench reference for biologists in our laboratory who are being trained in diatom identification. The guide also contains a glossary and generic key. The key was constructed with the beginner in mind, and is based entirely on the shape and markings of the diatom cell wall as observed in material mounted in hyrax. No attempt was made to place the taxa in their proper phylogenetic order. Laboratory personnel are encouraged to consult the taxonomic references for species identification.