The influence of sodium cholate on the crystallization of calcium oxalate from electrolyte solutions with a pH, ionic strength, temperature and concentrations of the constituent ions similar to those in urine has been studied. The additive has been chosen as a model for bile salts, which are considered to be the main factor responsible for lowering the urinary surface tension. Depending on the cholate concentration and consequently on its aggregation state in solution different effects on the overall crystallization process have been observed, Both in its molecular and partially associated state the additive inhibited growth and aggregation of the crystals by adsorption at the crystal/solution interface. In addition, micellar concentrations of the surfactant promoted crystallization of the metastable calcium oxalate dihydrate from solutions from which, without additive, calcium oxalate monohydrate was prevalently formed. The results of kinetic studies point to preferential inhibition of the thermodynamically stable monohydrate as the main reason for this effect.