Trichodesmium spp. fix atmospheric N-2 and so an element other than N limits production by these species in the oligotrophic ocean. Because dissolved inorganic phosphorus (DIP) is in short supply in many marine systems, it has been hypothesized that P is a growth-limiting nutrient for these species in nature. However, Trichodesmium is capable of hydrolyzing dissolved organic P (DOP) compounds and the inorganic products from hydrolysis may provide an additional source of P for growth. We investigated P dynamics and alkaline phosphatase activity in cultures and natural populations of Trichodesmium from the Atlantic Ocean and the north coast of Australia to determine whether hydrolysis of DOP could supply enough P to fuel growth. During the Atlantic cruise, concentrations of DIP were lower and chlorophyll (chl a)-specific rates of alkaline phosphatase activity by Trichodesmium were higher than during the Australian transect. However, because Trichodesmium were much more abundant during the Australian transect, where they represented the bulk of the surface chl a biomass, total water column rates of alkaline phosphatase activity were higher along the Australian transect than in the Atlantic, In both systems, DOP could potentially supply a significant portion of the cellular P necessary for growth. In cultures and natural populations, alkaline phosphatase activity was inhibited when DIP was present and increased in the presence of DOP. Cultures of Trichodesmium IMS101 grew equally well on media enriched with DOP or DIP at all but the highest enrichment levels. From these studies, we conclude that if the supply of appropriate DOP substrates is adequate, DOP can represent an important P source for Trichodesmium growth which should be included in estimates of P availability in surface waters.