Alkaline phosphatase activity (APA) was investigated in a representative population of Corallina elongata, common calcareous red alga iii the Mediterranean Sea. using p-nitrophenyl phosphate (pNPP) as a model substrate. A total of 66% of the phosphate cleaved from the substrate was immediately taken up by the algae, which may suggest a connection between APA arid phosphate uptake. APA kinetic exhibited an apparent negative cooperativity for the hydrolysis of pNPP. APA sites were located by a histochemical method. The staining technique showed that phosphatase activity was located on the monolayer of cortical cells (epithallial cells). The influence of factors important in nature on enzymatic activity was tested. In a range according with natural conditions, temperature, pH, salinity and photon irradiance influenced APA significantly. However, external phosphate showed little effect. The relationship between APA and the physical and chemical environment of the algae is discussed.