A unique aggregation behavior of potassium N-tetradecanoyl phenylalaninate in dilute aqueous solution was investigated. Surface tension, fluorescence intensity (using probes) and size distribution measurements suggest that a transition from vesicle to micelle takes place. It is presumed that formation of an acid-soap dimer, which is stabilized by an interaction between phenyl groups, brings about construction of vesicle particles at a very dilute concentration.