The waters of the Gulf of California are fabulously rich in marine life; they fairly teem with multitudes of fish of both commercial and sport varieties. To maintain these large numbers there must be correspondingly huge crops of their ultimate food-the phytoplankton. And, indeed, it was the enormous quantity of these microscopic plants that gave the Gulf its early and descriptive name-the Vermilion Sea. In 1939 and again in 1940 the "E. W. Scripps," research vessel of the Scripps Institution of Oceanography of the University of California, spent several months in these waters and among the data collected on these cruises were quantitative samples of this phytoplankton. It is with these samples that the present report deals.