ABSTRACT: During 2004 and 2006, alkaline phosphatase activity (APA) was measured in the northern Adriatic to establish whether it is an important mechanism for phosphorus recycling in this area, which is poor in orthophosphate (PO₄). From May to September, in conditions of exhausted PO₄, microbes in upper waters induced high APA (0.41 to 5.55 µmol l^–1 h^–1) to obtain phosphorus from the dissolved organic pool. The highest APA was found during spring and summer freshets due to the strongly deficient PO₄ versus inorganic nitrogen supply from the Po River. During the summer months, APA was lower yet still important for phosphorus recycling. In these months, the low Po River nitrogen supply resulted in lower microbial biomass and, consequently, lower APA. From October to December, APA in upper waters was low (<0.2 µmol l^–1 h^–1), even during large microbial blooms. During autumn blooms, PO₄ was probably supplied from deeper waters. In deeper waters, with an almost balanced inorganic N:P ratio for microbial requirements, APA was generally irrelevant during the entire investigation period. Our results emphasise the role of APA in meeting the seasonal microbial phosphorus requirements in the northern Adriatic.