The ultrasonic attenuation of skimmed milk, of the serum containing the soluble whey proteins and small casein aggregates and of ultrafiltered serum was measured in the frequency range 15–96 MHz. From these measurements the attenuation due to the colloidal casein micelles in serum was obtained. For the serum, α∝f1.86±0.02; for the soluble proteins, Δα∝f1.16±0.04; and for the casein micelles, Δα∝f1.06±0.05. The ultrasonic attenuation of the soluble proteins is similar to that of other protein solutions; however, ultrasonic attenuation of the casein micelles differed from that of the soluble proteins. Using the theory of Allegra and Hawley [J. Acoust. Soc. Am.36, 1545–1564 (1972)] for the ultrasonic attenuation of suspensions of elastic spheres, the expected attenuation for a model of the casein micelle suspension was computed. The frequency‐dependent attenuation, α’(f ), of the spheres is taken as an unknown for this model and, by varying α’(f ), a fit between the experimental data and the predictions of the theory was obtained. This fit requires the attenuation of the micelles to vary asf1.40±0.10and the micellar attenuation to be higher than for a typical protein solution of the same concentration. This suggests that relaxation processes of the casein polypeptide chains in the casein micelles differ from those of most soluble proteins studied so far by ultrasonic attenuation measurements. This conclusion is consistent with results from nuclear magnetic resonance spectroscopy.