Аннотация:The effective strength of the longitudinal magnetic field of the supergiant Gamma Cyg was measured in 1977 and 1981 using the photoelectric scanner-magnetometer of the 2.6-meter reflecting telescope of the Crimean Observatory. Beta CrB, a star with a known variable magnetic field, was chosen as a control star. Results indicate that Gamma Cyg may be characterized by periodic variations of the longitudinal field from + 40 to - 40 G with a period of 8.2594 d.