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The crystal growth of calcium oxalate trihydrate has been studied in supersaturated solutions at 37C under conditions in which the activities of calcium and oxalate ions were maintained constant by the potentiostatically controlled addition of titrant solutions containing these ions. It has been possible to stabilize calcium oxalate trihydrate sufficiently without transformation to the monohydrate, to establish the kinetic behavior. The rate of growth is proportional to the square of the relative supersaturation suggesting a spiral dislocation growth mechanism. Moreover, calcium oxalate trihydrate grows considerably more rapidly than does the monohydrate. Polyphosphate ion inhibits calcium oxalate monohydrate to a much greater extent than trihydrate. In the latter case, the reduction in growth rate in the presence of this ion can be interpreted in terms of a Langmuir isotherm. The stabilization of calcium oxalate trihydrate in the presence of inhibitors by preventing the formation of the transformation product, calcium oxalate monohydrate, may be an important factor in the development of renal stones.