Deep or subsurface chlorophyll-a maximum (DCM) was studied at one station in the middle Adriatic from December 1996 to June 1998. Chlorophyll-a concentration, abundance, volume carbon concentration, size-fraction of phytoplankton and phytoplankton community structure were determined. In addition, physical and chemical factors as well as nutrients were determined. The DCM occurs during both the vertical mixing and stratification period in the middle Adriatic Sea. It is most frequent between 50 and 75 m. It is located below the pycnocline and it is associated with the nutricline. Phytoplankton size-fraction and community structure vary seasonally. The DCM is most pronounced during spring phytoplankton blooms with diatom dominance. Procaryotic picoplankton Synechococcus sp. was abundant in DCM during summer stratification. The DCM represents both a biomass maximum and a phytoplankton adaptation to low irradiance.