A quantitative analysis of calcareous nannofossil assemblages was carried out, with an optical microscope, on four deep-sea cores raised from the eastern Mediterranean and selected to form a NW-SE transect. A total of 748 samples were studied, including a very detailed analysis of sapropels S5 and S6 in two, out of the four cores. This paper is restricted to the discussion of the results obtained on Florisphaera profunda which provided important insights on the mechanisms of sapropel formation, an intricated and unravelled puzzle for the scientists of the last decades. On the basis of abundance variations of F. profunda, the only species of calcareous nannoplankton which inhabits the lower part of the photic zone, I suggest that sapropel formation was triggered by increased primary production, either confined to a Deep Chlorophyll Maximum (DCM) (a concept previously introduced by Rohling and Gieskes, ) or extended to a thicker layer of the photic zone. When increased primary production was confined to the DCM (sapropel S3, S4, S5, S7, and S1, this latter only in the eastern part of the basin), the abundance of F. profunda was enhanced greatly with respect to all other species living in the higher part of the photic zone. On the contrary, when the increased primary production was extended to a thicker layer of the photic zone (“glacial” sapropels S6 and S8) the production of most nannoplankton species was favored, decreasing the relative proportion of F. profunda.