Tetrahymenu pyriformis W suspended in a buffered glucose solution accumulated orthophosphate [3*P] from the external solution at a measurable rate. The uptake of orthophosphate by the organisms was linear with respect to time when corrections were made to account for a constant efflux observed during the one hour time course of the experiments. Such corrections were based on the measured lowering of the relative specific activity of the suspension medium and led to the derivation of the expressions for the influx and efflux coefficients. The derived expressions for the coefficients are based solely upon the isotopic measurements and by means of these equations it is possible to describe the observed net inward flux of orthophosphate in quantitative terms. The dependence of the uptake of orthophosphate on the external concentration of orthophosphate followed Michaelis-Menten kinetics, and the temperature coefficient (l8-2Sc) of 1.7 for the process fell into the range n o r m a y associated with a chemical reaction. The kinetic pattern, per se, does not distinguish a membrane transport mechanism from metabolic incorporation of Pi. Deviations from the expected pattern of uptake were observed at low temperatures.