The critical micelle concentrations (CMC) and micellar weights of decyl- and dodecylbetaines were determined by surface tension and light-scattering measurements. These values and those determined previously for octylbetaine revealed the following.1.The logarithm of CMC decreases linearly with the number of carbon atoms in the alkyl group. The slope of log CMC plotted against the carbon number of alkyl chain is nearly equal to that obtained for nonionic surfactant, and steeper than that for ionic surfactant.2.The logarithm of association number increases linearly with the number of carbon atoms in the alkyl group. The slope of the straight line obtained is larger than that for ionic surfactant.