SUMMARYAn extractive procedure for detection of surplus‐stored phosphorus (luxury consumption) in algae and an enzymatic analysis for conditions of P‐limited growth in algae have been evaluated. A simple 60‐min boiling water extraction of algae known to contain surplus P separates essential P compounds and surplus‐stored P compounds. Surplus P compounds can be measured in the extract as orthophosphate. Extracts of algae limited in their growth by the amount of available P contain little or no orthophosphate. Limitation of algal growth by P supply induces the enzyme alkaline phosphatase. The activity of this enzyme can be measured at pH 9 using p‐nitro‐phenylphosphate as substrate. Algae which were P‐limited and contained no extractable orthophosphate have as much as 25 times more alkaline phosphatase activity than algae with surplu