ABSTRACT To describe potential intraspecific variation in phosphorus incorporation in two strains of Phaeodactylum tricornutum (Bohlin), Ub3 and Ub7, alkaline phosphatase (AP) activity was evaluated via enzyme-labeled fluorescence assay. Analysis using the probe ELF-97(r) provides individual evaluation, and therefore can determine the nutritional status of inorganic phosphorus in phytoplanktonic cells. Bioassays compared the control treatment to both phosphate-enriched and phosphate-depleted treatments by varying only the phosphate concentration in the media. The P. tricornutum strains exhibited differences in their development when incubated in the phosphate-enriched media. The development of the Ub7 strain differed by exhibiting "luxury uptake" and utilization of organic phosphorus, and the alkaline phosphatase analysis indicated limitations of this clone under such conditions. The Ub7 strain showed higher AP activity, when compared to Ub3, in the P-enriched condition. P. tricornutum presented increases in AP activity and low variation in Surface/Volume ratio, by increasing biovolume and its maximum linear dimension, as strategies for phosphate incorporation. Our results highlight intraspecific differences in alkaline phosphatase activity, and hence differences in the incorporation of organic phosphorus, as the tested species regulated enzymatic activity under different external phosphate concentrations.