An electromagnetic rotating adjuster that has a servomotor with a can-shaped rotor, which is supported by means of a shaft via a first roller bearing and a second roller bearing, in which the disposition of the roller bearings in the one piece housing is inaccessible. For axial fixation of one second roller bearing, a spring washer is disposed in a second bearing opening and the second roller bearing is pressed against it, the outer race of the bearing having a detent groove on its circumference which groove is engaged by resilient detent tongues of a detent ring, supported in an annular groove of the housing. This assures secure fixation of the second roller bearing in the axial direction in a way that both suits the needs of large-scale mass production and is economical. The electromagnetic rotating adjuster may be used in control of the idling air in internal combustion engines. The resilient tongues and washer facilitate removal, as well as installation, of the rotor assembly.