T he composition and the contribution of the 4 different cell size classes (>100, 50-100, 20-50 and <20 nm) of the phytoplankton population in the neritic water of Alexandria were studied over an annual cycle. Their abundance exhibited considerable variability in times. Nanoplankton contributed numerically most of the total production (58.32%, range of 4.34-95.7%), culminating its peaks during the warm seasons, while the centric diatom, Skeletonema costatum and the microflagellates, Pyramimonas sp. and Micromonas sp. were its major constituents. Netplankton dominated at high nutrient levels. However, seasonal shifts were evident. The floristic data indicated the predominance of large diatoms and dinoflagellate cells at times, influencing their relative importance to the over all standing crop population. Temperature was significantly correlated with all the size classes, except that of >100 \im 9 which seems a phosphate dependent. Salinity and silicate concentrations seem affecting the variability of the smaller size classes.