The chlorophyll-a data set established by Turkey, Ukraine, Russia, Bulgaria and Romania was evaluated for the 1990-1996 period and only the surface chlorophyll-a was evaluated and interpreted for different parts of the Black Sea. Three well defined chlorophyll-a maxima were observed in the surface waters in the shelf region: a winter maximum in January-February, a spring-early summer one in May-June and an autumn peak in September-November. One of the expected results was the high level of chlorophyll-a concentration in shelf waters (0.02-34.0 mu gL(-1) as the range of monthly averages) compared to deep regions (0.02-2.5 mu gL(-1)) throughout the whole year. Chlorophyll-a data related to deeper central part of the Black Sea showed different and unusual trends in terms of seasonal and interannual variability's. A late winter blooming continued with slightly decreasing trend till late spring which collapsed during May. The developments of moderate mid-summer and autumn blooms were also observed. A very unusual development of phytoplankton population in 1992 mid-summer caused formation of a chlorophyll-a peak which was almost at the same order of magnitude with winter-spring bloom.