Out of 502 epileptics studied, 54 patients showed the mu-rhythm on the EEG which was more pronounced on the side of the epileptic focus. It was usually sharply shaped and often attended by the central beta-rhythm. The diagnostic value of the paroxysmal mu-rhythm is evident although the latter occurs rather rarely. When the epileptic foci were located in the temporal lobe, the mu-rhythm was more disorganized as compared with the frontal-central localization. The authors failed to detect these characteristics of the mu-rhythm in 33 control subjects. The findings obtained may be used as an auxiliary topical-diagnostic criterion, particularly in cases where the EEG exhibits no epileptic phenomena in the interictal period.