During June 1997 cruise by the R/V Science No.1, observations on temporal and spatial variations of size-fractionated phytoplankton standing stock and primary production were carried out in the Bohai Sea. Size-fractionated chlorophyll a and primary production, PAR (photosynthetically available radiation), as well as the related physico-oceanographic and zooplanktonic parameters were measured at five time-series stations representing sub-areas of the sea. Results obtained showed that there were marked spatial zonation of chlorophyll a and primary production in the Bohai Sea. The values in Laizhou Bay, Liaodong Bay and Bohai Bay were high and appeared closely related with tidal fluctuations, i.e. high chlorophyll a concentration occurred during high tide in Laizhou Bay, and during low tide in Liaodong Bay and Bohai Bay. In the mouth and the central region of the Bohai Sea, the values were relatively low and no relationship with tidal fluctuation could be found. Chlorophyll a concentration vertically decreased from surface to bottom in Liaodong Bay and Bohai Bay, while it increased in Laizhou Bay, the mouth, the central region of Bohai Sea, and maximized at the bottom. Size-fractionation results showed that nano-and picoplankton (< 20μm) predominated in the Bohai Sea phytoplankton communities during late spring. The average contribution to total chlorophyll a in each station was 76%-95% (mean=87%). The contribution of net (>20μm), nano-(2-20μm) and picoplankton (<2μm) were 13%, 63% and 24% to total production, and 9%, 53% and 38% to total chlorophyll a, respectively. It proved the importance of nano-and picoplankton in phytoplankton communities in the Bohai Sea ecosystems. The paper also discussed the factors, such as light intensity, zooplankton grazing pressure, etc. governing standing stock and production of phytoplankton in the Bohai Sea.