Rhodopseudomonas capsulata required potassium (or rubidium or cesium as analogs of potassium) for growth. These cations were actively accumulated by the cells by a process following Michaelis-Menten saturation kinetics. The monovalent cation transport system had Km's of 0.2 mM K+, 0.5 mM Rb+, and 2.6 mM Cs+. The rates of uptake of substrates by the potassium transport system varied with the age of the culture, although the affinity constant for the substrates remained constant. The maximal velocity of uptake of K+ was lower in aerobically grown cells than in photosynthetically grown cells, although the Km's for K+ and for Rb+ were about the same.