The phosphate-limited growth kinetics of Rhodotorula rubra, a small yeast of marine origin, were examined by analysis of 32P distributions in continuous cultures. Isotope relaxation procedures were used to identify unidirectional flows of Pi and organic phosphate among compartments modeled during growth. The concentrations of phosphates in these compartments at various growth rates were used, together with attendant flows, to produce a mathematical model of growth. Both Pi and phosphate-containing metabolic intermediates leaked from cells during growth. Total leakage ranged from 4 to 10% of influx and was comprised mostly of Pi. Transport capacity was at least 10 times that required for growth at saturating Pi concentrations, so that influx was linear with concentration during growth. This led to the realization that the curvature of Monod plots (Kmu = 12 nM mumax = 0.18/h, and the threshold At = 2.5 nM) is due to change in yield with growth rate. Growth rate related to Pi by the affinity, aA (= 0.43 liter/mg of cells.h) of cells for Pi and the growth rate-dependent yield. It was also specified by a series of kinetic constants that specified flow among the various compartments and equilibrium compartment concentrations as they were set by extracellular Pi. The importance of leakage by healthy cells to the organic chemistry of aquatic systems is noted.