Studies on phosphatases (acid and alkaline) inSynechococcus aeruginosus, a prokaryotic blue-green alga, andScenedesmus incrassatulus, an eukaryotic green alga under different conditions revealed that the acid phosphatase exhibited maximum activity to pH 4·7 and 37°C in both the algae while alkaline phosphatase displayed greatest activity at 37·5°C and 10 pH inSynechococcus aeruginosus and at 10·6 pH and 37·5°C inScenedesmus incrassatulus. TheKm values were found to be 50 and 17 mM for acid phosphatase and 4·2 and 8·3 mM for alkaline phosphatase in blue-green and green algae respectively. The inhibitory effect of metal ions (Cu2+, Fe2+, Hg2+, Zn2+) on acid phosphatase activity is similar in both the algae whereas on alkaline phosphatase, their influence varies. Sodium fluoride and EDTA inhibited both the phosphatases activity with the increased concentrations in both the algae. Benzene hexachloride (50%) retarded these enzyme’s activities at higher concentrations in these algae.