For more than twenty years, investigations have been conducted at the Scripps Institution of Oceanography on the distribution and abundance of marine plankton diatoms at several stations along the Pacific coast from Scotch Cap, Alaska, to La Jolla, California. Besides these regular series of collections, numerous short series have been taken on cruises of the research vessel of the Scripps Institution, Navy vessels, and private yachts, covering routes from Alaska to Callao, Peru, and in the Gulf of California. In the course of examining the thousands of samples collected, many perplexing problems of identification of species have arisen. In an effort to help solve some of these problems and to fill a need for a taxonomic paper on the plankton diatoms of our region, a need evident from the many requests received at the Scripps Institution, the present paper has been prepared. Although the main emphasis has been placed on the pelagic diatoms found in waters off southern California, pelagic species present along the Pacific coast from Alaska to the Canal Zone and in the Gulf of California, as well as some littoral species frequently found in plankton collections, have been included. A small number of species listed on the Scripps Institution records but not verified by the author have been omitted, and without doubt some species have been overlooked. A serious attempt has been made to include a large number of drawings of many species to show variations that commonly occur in size and structure, since much of the confusion in quantitative enumeration arises because of the departure of species from their “typical” appearance as sketched in the usual papers and books on the diatoms. Only a brief general discussion of the group as a whole has been included. The aim has been to produce a usable manual for the rapid and easy identification of species commonly found in plankton collections.