Measurements of the dynamic surface properties are used for the first time for the determination of the mean aggregation number of micelles. An approximate expression for the dilational dynamic surface elasticity of micellar solutions in the concentration region near the CMC is obtained on the basis of the more general theory which has been developed several years ago (B. A. Noskov, Izv. Akad Nauk SSSR. Mekh Zydkasty i gaza N2, 105 (1989)). If the reverse frequency of external perturbations does not exceed the characteristic time of the slow step of the micellization process, the only properties of micelles which determine the dynamic surface elasticity are the mean aggregation number and the mean diffusion coefficient. The method of longitudinal surface waves has been applied to determine the dynamic surface elasticity of solutions of dodecyldimethyl phosphine oxide in the frequency range from 0.2 to 1.8 Hz. Both the real and the imaginary components of the dynamic surface elasticity depend nonmonotonically on the concentration below the CMC. All the experimental data agree reasonably well with the pure diffusional adsorption mechanism. Above the CMC both components of the surface elasticity decrease faster with the surfactant concentration. This fact can be explained by the contribution of micelles to the mass exchange between the surface layer and the bulk phase. Application of the theory of dynamic surface properties of micellar solutions to experimental data all offs us to obtain an estimation of the mean aggregation number of micelles of dodecyldimethyl phosphine oxide.