Cell volumes and per cell carbon contents of different diatoms (10 centric, 14 pennate and 1 dinoflagellate) collected from Dona Paula Bay in the central west coast of India have been analyzed. Morphometric information on the phytoplankton types recovered through many replicate samples for calculating the cell volumes and to estimate per cell carbon content has been presented. The data on the above aspects are of of great significance for instance for comparison of data from different geographic locations. Cell dimensions of all identified plankton species comprising diatoms (90% of total cell counts), dinoflagellates (1%) and others (9% of total cell counts) were recorded for calculating per cell carbon content of the observed species. The carbon content, while correlating positively with increasing cell size of phytoplankton species recorded, was found to be directly dependant on cell volume. The present study elucidates that the use of a standardized species list with fixed size-classes and biovolume will be necessary for a realistic measurement of carbon contents. The present study also suggests that morphometric measurements are necessary for obtaining dependable estimations of cell volumes and carbon contents