Present study focuses on the cell volume and diversity of phytoplankton, which are unique indicators of aquatic salinity. Cell volume and Shannon Weiner Species Diversity Index values of the documented phytoplankton in two significantly different salinity sectors in the Indian Sundarbans: western and central were compared. Western sector exhibited higher Shannon Weiner Species Diversity Index value (mean Ĥ value = 3.2715) compared to that of the central sector (mean Ĥ value = 3.2504). Cell volume of the observed species was also more in the western sector (118.83 μm3 to 304322.23 μm3) compared to the central sector (114.23 μm3 to 295199.31 μm3), thus indicating the regulatory role of salinity on the free floating producer community in the aquatic ecosystem. ANOVA results indicate significant differences (p < 0.05) in phytoplankton diversity and cell volume between the western and central Indian Sundarbans. © 2014, National Institute of Science Communication and Information Resources (NISCAIR). All rights reserved.