Microviscosity, micellar aggregation number, and the critical micelle concentration (cmc) of potassium N-acylalaninates were determined in KCl solutions by fluorescence probe methods together with their solution viscosity. Two inflection points were observed on the plot of microviscosity vs KCl concentration. The first and second inflection points were attributed to the beginning of micellar growth and intermicellar interaction, respectively. At a KCl concentration above the first inflection point, the micellar aggregation number steeply increased and the log-log plot of cme against KCI concentration deviated downward from a linear Corrin-Harkins relation. Above the second inflection point, the micellar solutions of N-acylalaninates became very viscous and exhibited a constant microviscosity. Comparison with the previous data for N-acylvalinates (Langmuir 1995, 11, 2951) revealed that microviscosity on the micellar surface was larger in N-acylalaninates than in N-acylvalinates, while microviscosity in the micellar core was the same in the two surfactant series. Each N-acylalaninate solution was divided into four regions (solution of monomer surfactant, region of globular micelles, region comprising large micelles followed by micellar growth, and region where there is strong intermicellar interaction). Each boundary between the regions shifted to lower concentrations of KCI and N-acylalaninate as the acyl chain became long.