Culturable heterotrophic bacterial and phytoplanktonic densities were investigated at four sites in the Southern Adriatic Sea (Brindisi, S. Cataldo, Otranto and S. M. di Leuca) over an annual cycle. The main phytoplankton groups, the bacterial biodiversity, as well as the faecal contamination indicators were determined. Culturable bacterial numbers averaged 4.8 +/- 0.2 log CFU ml(-1) whereas phytoplankton numbers averaged 2.1 +/- 0.4 log cells ml(-1). Relationships between culturable bacteria, phytoplankton and the environmental factors were established. Bacterial and phytoplankton densities usually depended significantly on temperature, dissolved oxygen, phosphate and nitrite only in the S. Cataldo transect. In all the examined transects phytoplankton showed a bloom during the January-February period followed by a bacterial peak during the February-March period. Thus we can suppose that the phytoplankton winter bloom is responsible for the availabily of organic matter for bacterial populations in the following months in this oligotrophic ecosystem.