The dynamic mobility and dielectric response of Na-bentonite have been measured at megahertz frequencies. The particles consist of thin, approximately hexagonal plates. Information about the particle size and the electrokinetic charge or zeta potential can be obtained from the dynamic mobility spectrum. Interpretation of the dynamic mobility spectrum is complicated by the presence of anomalously high surface conductance, as also occurs in the case of kaolinite. This profoundly affects the shape of the dynamic mobility spectrum at low (<5 X 10(-3) M) electrolyte concentrations. The approximate formula derived by Loewenberg to take account of the effect is found to be in excellent agreement with the results, the agreement being rather better than that found in the case of kaolinite. The surface conductances of the particles, determined directly by dielectric response measurements, compare almost quantitatively with those needed to fit the dynamic mobility data using the theory described above. The results also suggest that the ratio of the mobility of ions in the Stern layer compared with those in the diffuse double layer (at the high frequencies involved here) is about 0.6 for bentonite, which is similar to the value for kaolinite.