The critical micelle concentrations of aqueous solutions of N alpha-acyl-L-histidine have been determined by the spectral shift method with Rhodamine 6G and by the light scattering method. With the spectral shift method critical micelle concentrations of 40, 9.0, 1.0, 0.11, and 0.012 mM were obtained for N alpha-acyl-L-histidine containing saturated acids of 8, 10, 12, 14, and 16 carbons respectively, at 45 degrees C and pH 8.6 in the absence of added salt. For the homologs containing 10, 12, and 14 carbon acids, critical micelle concentration of 9.0, 1.0 and 0.11 mM were determined by the light scattering method. The light scattering studies yield micelle weights of 60, 66, and 84 thousand for the C-10, C-12, and C-14 homologs, respectively. N alpha-acyl-L-histidine is an unusual surfactant in that the hydrophilic portion of the molecule is relatively large and contains both an ionic group (carboxylate group) and a nonionic group (imidazole side-chain). The bulky hydrophilic group of N alpha-acyl-L-histidine causes this molecule to exhibit physico-chemical behavior which is not typical of that exhibited by most ionic surfactants. In particular, the dependence of the critical micelle concentration on the acyl chain length and on the concentration of added salt is atypical. Chemical shift measurements (by NMR) on the C-2 and C-5 protons of imidazole in micellar N alpha-dodecanoyl-L-histidine indicate that the imidazole group is, indeed, positioned at the water-micelle interface.