The translocation of phosphorus as 32P was investigated in two species of rooted, submerged, aquatic angiosperm, Potamogeton pectinatus and Potamogeton crispus. Experiments were conducted in two forms of partition apparatus for periods of either 1 to 48 h or 6 to 10 days. The apparatus used for the shorter-term experiments incorporated three compartments so that possible isotope leakage through seals could be critically distinguished from plant excretion. Results of autoradiography and of radioactive counting provide evidence for a circulation of 32P in these plants akin to that described for terrestrial plants but with slower rates of movement. Quantitative differences in rates and amounts of 2P transport in the two species are discussed in relation to their anatomy and ecological distribution. There was no evidence of excretion of isotope, following transport, in the experiments conducted.