At 25 C ultrasonic attenuation spectra between 100 kHz and 400 MHz as well as sound velocities and densities of aqueous solutions of the surfactant n-octyl-beta-D-glucopyranoside and of the cage compound a-cyclodextrin have been measured. The liquid system reveals a critical association concentration (cac) exceeding the cmc of the surfactant almost by the cyclodextrin concentration and thus indicating a significant formation of cyclodextrin-surfactant inclusion complexes. The ultrasonic spectra display altogether four relaxation regions. The one with largest relaxation time (0.27 mu s <= tau(1) <= 1.6 mu s) exhibits a noticeable amplitude at surfactant concentrations larger than the cac only. It is assigned to the monomer exchange between the micelles and the suspending phase. A term with relaxation time tau(2) (33 ns <= tau(2) <= 135 ns) is characteristic for solutions containing both solutes. It is assumed to reflect the inclusion complex formation. Complexes with 1:1, 2:1, and 1:2 stoichiometry, appear to exist. The terms at higher frequencies (4.8 ns <= tau(3) <= 9.8 ns; 0.8 ns <= tau(4) <= 2 ns) are due to fluctuations of the carbohydrate residues around the glucosidic bond angles and to the rotational isomerization of the exocyclic hydroxymethyl groups, respectively.