To describe potential intraspecifi c variation in phosphorus incorporation in two strains of Phaeodactylum tricornutum (Bohlin), Ub3 and Ub7, alkaline phosphatase (AP) activity was evaluated via enzyme-labeled fl uorescence assay. Analysis using the probe ELF-97 ® provides individual evaluation, and therefore can determine the nutritional status of inorganic phosphorus in phytoplanktonic cells. Bioassays compared the control treatment to both phosphateenriched and phosphate-depleted treatments by varying only the phosphate concentration in the media. Th e P. tricornutum strains exhibited diff erences in their development when incubated in the phosphate-enriched media. Th e development of the Ub7 strain diff ered by exhibiting "luxury uptake" and utilization of organic phosphorus, and the alkaline phosphatase analysis indicated limitations of this clone under such conditions. Th e 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 intraspecifi c diff erences in alkaline phosphatase activity, and hence diff erences in the incorporation of organic phosphorus, as the tested species regulated enzymatic activity under diff erent external phosphate concentrations.