Two expeditions in the northeastern Mediterranean by the R/V Bilim (in July 1988 and March 1989) scanned an area of about 3 x 10(5) km2 to determine, in situ, the relative fluorescence intensities of the upper layer waters. The in situ fluorescence intensities exhibited a fair correlation with the discrete chlorophyll-a concentrations when the concentrations exceeded 0.1 mug/L. Light intensities indicated that the euphotic zone had an average thickness of 100 m in the open waters. The deep chlorophyll-a maxima (DCM) at the bottom of the euphotic zone usually coincided with the maxima observed by in situ fluorometry and were a prevalent characteristic of the oligotrophic northeastern Mediterranean. The formation, maintenance and location of the DCM were controlled by the changes in light attenuation and nutrient concentrations occurring in the anticyclonic and cyclonic gyres. Accordingly, DCM with relatively high chlorophyll concentration formed at shallower depths in late winter (e.g. 50 m for March, 1989) whilst in summer DCM possessed lower chlorophyll concentrations and were found as deep as 100 m in the anticyclonic regions. Although the depths of the maximum fluorescence intensity varied in space and time, they remained within a relatively narrow range of isopycnal surfaces, namely, from 28.8 to 29.0 in March 1989, and from 28.6 to 29.0 in July 1988; the appearance of maximum fluorescence intensities at larger density values but at shallower depths in late winter is principally the result of lower light intensity, available nutrients and hydrological changes in the upper layer.