56 samples were collected from 14 stations along the Spanish coast, the North African coast, the Balearic Islands and Sardinia. 60 species of diatoms were recorded made up mostly of tychopelagic and benthic forms; these were far more abundant than the Dinophyceae of which only 22 species were recorded. 6 species of phytoplankton were dominant and represented similar to 70% of the total samples. The average content of chlorophyll a at depths of 1-70 m was 0.35 mg /m super(3), that of chlorophyll b was 0.15 mg/m super(3) and that of chlorophyll c was 2.20 m-S.P.U./M super(3)/hour. The highest level of chlorophyll a corresponded to regions of very low phosphate levels and the number of cells and species were at a maximum for an incident light of 10-15% and at a maximum for 5%. The distribution of O sub(2) in the euphotic layers, the distribution of diatoms and the production level appeared to be related. The maximum production was often recorded at levels below the thermocline.