Extracellular alkaline phosphatase activity (APA) was detected in the marine macroalga Ulva lactuca, cultivated in laboratory conditions under phosphate limitation. Algal discs were grown in continuous light and in darkness for a maximum of 205 h in sterilized seawater supplied with ammonium nitrate and various phosphate concentrations. APA and phosphate uptake in U. lactuca was stimulated in light whereas inhibition was detected in darkness. APA increased significantly during the cultivation in a phosphate-deficient medium but was suppressed at high external phosphate concentrations. In addition, pretreatment in penicillin over 48 h markedly enhanced enzyme activity during P limitation. The pretreatment in penicillin reduced the number of surface-associated bacteria. Our results indicate that APA in U. lactuca was dependent on the external phosphate concentration, energy from photophosphorylation and bacteria. Since intracellular polyphosphates and an orthophosphate pool still were displayed in 31P NMR spectra after cultivation at phosphate-limiting conditions, P deficiency was not suggested. Unfortunately, NMR spectra reflect total amounts of P compounds and local differences in the P status of the multicellular tissue could not be distinguished. Postulating a negative correlation between the P status and APA, a lower P status in meristematic areas during P limitation was suggested. Alkalisation of the growth medium during algal growth in light, and/or intracellular pH changes may additionally have affected enzyme production.