Abstract The submerged aquatic freshwater macrophyte Isoetes australis S. Williams grows in rock pools situated in south-western Australia, an environment where dissolved inorganic phosphorus (Pi) availability possibly limits growth. In contrast to the two coexisting aquatic species, Glossostigma drummundii and Crassula natans, I. australis did not form relationships with mycorrhiza. Pi uptake kinetics were determined for I. australis in experiments using radioactive 33Pi. Roots had a higher Pi affinity (lower Km) than leaves, but roots also had a lower Vmax, which is discussed in relation to the low ambient Pi concentrations. I. australis showed morphological adaptation which could relate to the low Pi environment by having approximately twice as much root tissue as leaf tissue (by dry mass), facilitating access to the higher P pools in the sediment compared with the shallow water column. A short-term translocation experiment revealed high amounts of Pi translocation internally in the plant which seemed to go from roots and oldest leaves to younger leaves. As a result of the high root to shoot ratio, high surface area, root uptake kinetics, and sediment Pi availability, roots accounted for 87% of plant Pi uptake and the green parts of the leaves for about the remaining 13%. As a result the estimated P budget for the rock pools showed that the surface water had a Pi turnover of about 1.5 days, whereas the porewater Pi pool was renewed about 10 times per day to satisfy the P requirements of I. australis.