The sensitivity of the resolving power of electron microscopes to ambient AC magnetic stray fields is well known. The highest permissible values of stray fields are fairly low and it may be difficult to find a suitable site for a microscope in normal laboratory conditions. A brief description of a device is given which reduces the disturbing effect of these stray fields by the method of automatic compensation. The device was tested on a JEM 6A transmission microscope. Even in an AC field as high as 24 mG (peak to peak) substantially higher resolving power was attained than that guaranteed by the manufacturer. Possible applications of the device to other electron beam instruments and in different magnetic stray fields are briefly discussed.