Phytoplankton biomass has been studied in the Levant Sea (Eastern Mediterranean) during summer 1983 in order to picture its vertical distribution as well as its limitation mechanisms. Biological tests carried out on the natural populations of the Basin show that P is the first biomass-limiting element and that N is the second. The simultaneous input of P and N, however, is not sufficient to induce growth; this suggests that Mn has an essential role in the phytoplanktonic growth in this ultra-oligotrophic water. Discussion deals with the significance of the deep chlorophyll a maximum and with the physiological processes allowing it to last.