Primary productivity was measured monthly in Lake Qarun during 1993 using C technique and the annual average value was 105.5 mg Cm" h" . The study revealed that both biotic and abiotic variables have affected the primary productivity. Phytoplankton biomass, as biotic factor, was represented by total Chlorophyll a (annual average of 26 mg m"3). Chlorophyll a was fractionated into two major groups based on their availability to herbivorous zooplankton; nanoplankton (<20p.) or edible algae and netplankton (> 20fi) or inedible algae. Nanoplankton was dominated and represented by about 69,4% of total phytoplankton, while netplankton was about 30.6%. Size fraction ratio was controlled by their relative size-specific growth rates not by herbivorous zooplankton. There was a linear relationship between the relative contribution of nanoplankton and total algal biomass (r=0.96, p>95). Primary productivity has been affected by nanoplankton more than any other biotic variable.Abiotic parameters (physical and chemical) were also studied and discussed in relation to its effect on primary productivity. The statistical analysis cleared that water temperature and NO3-N were the main factors regulating primary productivity among all ecological parameters tested.