ABSTRACTConcentrations of hot‐water extractable phosphorus from most samples ofCladophora glomerata(L.) Kütz. were relatively high (0.06–0.68%) and correlated closely with total dissolved P in ambient Lake Michigan water.Cladophorawas able to hydrolyze polyphosphates by enzymes found in intracellular, extracellular and cell wall fractions. The intracellular phosphatase activity is pH dependent with the optimal hydrolysis rate at pH 7.8. Secretion of phosphatases is affected by pH, with maximum rate at 7, but affected little by light intensity. Magnesium is the most effective metallic cofactor required for maximal rates of intracellular phosphatase activi