Effects of different photoperiods on growth of Thalassiosira fluviatilis and Skeletonema costatum revealed that both the species preferred longer duration of light for their maximum growth. The growth of T. fluviatilis was higher than that of S. costatum in different photoperiods. In T. fluviatilis maximum growth was observed at continuous light (24 hrs) and in S. costatum at 20.4 LD cycles. Short term exposure to light like 8 and 6 hrs did not enhance good growth in both the species.