We have studied the effects of a phospholipid (DL-alpha -phosphatidylcholine dimyristoyl: DMPC), which forms an extended flat bilayer aggregate (nearly zero spontaneous curvature), on the spherical-cylindrical shape of nonionic surfactant micelles (pentaethylene glycol mono-n-dodecyl ether; C12E5) in terms of an aggregation number, critical micellization concentration (cmc), second virial coefficient (A(2)), and hydrodynamic diameter (D-H) by laser light scattering. The aggregation number and critical micellization concentration increase as the amount of DMPC is increased. On the basis of the cmc values, the total free energy per micelle of C12E5/DMPC mixtures is estimated. The free energy per micelle of C12E5/DMPC mixtures decreases as DMPC is added. This is consistent with the decrease of A(2) and the strong hydrophobicity of DMPC compared with that of C12E5. The average contour length, the diffusion coefficient, and the end-cap energy of mixed micelles are estimated on the basis of the cmc and molecular specific volumes of the moiety. The end-cap energy of the mixed micelles and the average contour length increase as DMPC is added, which is also reasonable considering the molecular structure of DMPC. Furthermore, the diffusion coefficients obtained from dynamic laser light scattering are in excellent agreement with the estimated diffusion coefficients obtained from a one-dimensional growth model based on static light scattering measurements.