The characteristic growth pattern, chlorophyll concentration and productivity of a cyanobacterium Synechocystis salina and a diatom Chaetoceros affinis have been studied separately. The effect of S, salina on the growth, pigments and primary productivity of C. affinis has been assessed. An inhibitory effect has been noticed in the growth, concentration of chlorophyll a and c , and in the net production while an enhancing effect has been observed in the phaeophytin concentration of the diatom. The mutual impact of the species of two different classes has been assessed in their dispecific culture under the same physicochemical parameters. An overall inhibition on the growth, chlorophyll concentration and productivity was observed.