Micelle aggregation number m of dodecyldimethylamine oxide was determined as a function of the micelle composition or the degree of ionization of the micelle alpha(M) in 0.1 and 0.2 M NaCl at 25 degrees C with light scattering and the steady state fluorescence probe method. The two methods gave consistent results in most cases. The aggregation numbers at the critical micelle concentration (cmc) were nearly identical for both cationic and nonionic micelles : about 70 and 80 in 0.1 and 0.2 hi NaCl, respectively. The aggregation number increased as alpha(M) approached around 0.4-0.5. The maximum values were about 100 and 150 for 0.1 and 0.2 M NaCl. At intermediate compositions, the Rayleigh ratio of the scattered light increased with the concentration, and this increase was interpreted as the increase of micelle size with concentration; Apparent aggregation numbers at a finite concentration were evaluated on the assumption of negligible contribution from the second virial coefficient. The idea of the growth of micelles was supported by dynamic light scattering but not by the fluorescence probe method. From the observed variation of the aggregation number at cme with alpha(M), the width of the aggregation number distribution of nonionic micelles is estimated on the basis of a simple theory.