A study of the size structure of chlorophyll a (Chla) covering the major part of the equatorial Atlantic Ocean from 5 degree N to 5 degree S leads to the conclusion that seasonal nutrient enrichment in the open eastern equatorial Atlantic does not drastically affect the size distribution of the primary producers: 90% of the total Chla is everywhere contained in the < 20 mu m fraction on the average. In the coastal upwelling near Dakar, this percentage is less than 60%, for the same range of chla concentrations. In the equatorial region the percentage of < 1 mu m Chla is the same in the deep chlorophyll maximum located at the top of the nitracline during the warm season and in the subsuperficial maximum of the upwelling.