The nature of the premicellar state of sodium cholate formed at the early stages of the micelle formation process has been analyzed using pyrene as a fluorescent probe. Formation of the premicellar aggregates of cholate is dependent on a diffusion-controlled process, and stabilization of the premicellar aggregates of cholate molecules appears to require both hydrophobic and electrostatic interactions. In addition, it has been demonstrated that there is a close relationship between the efficiency of the premicellar aggregate formation and the value of the critical micelle concentration of cholate. From these observations, it is suggested that the premicellar aggregates serve as nuclei in the micelle formation process.