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Investigation of succinate dehydrogenase (SD) activity in the fibers of the frog sartorius muscle revealed three groups of muscle fibers (dark, pale, and intermediate). The SD activity in the muscle fibers was inversely proportional to their diameter. The auter surface of the sartorius muscle consists mainly of dark muscle fibers, the inner surface mainly of pale fibers. A microelectrode study showed that fibers of the outer surface have lower values of action potentials, a longer negative after-potential, a lower quantum composition of the end-plate potentials, and higher amplitude and lower frequency of miniature end-plate potentials than fibers of the inner surface. Analysis of the results reveals definite relations between the histochemical profile of frog sartorius muscle fibers and their electrophysiological characteristics.