Abstract: The insulating duct (6) is formed by a material which is built up in layers and is shaped in the manner of an L. The base of the L, which serves as the duct bottom (8), is of rigid design, whereas that limb of the L which serves as the duct wall (11) is of flexible design. A reinforcing insert (12, 13) is placed into the duct bottom (10, 11) and is guided into that part of the duct wall (10, 11) which adjoins the duct bottom (7, 8). In a U-shaped slot-insulating means formed from two of these insulating ducts (6) the radii located at the transitions between the base and the limbs of the U then exhibit great strength. During operation of a rotating, electrical machine into whose rotor a slot-insulating means produced in such a manner has been inserted, migration of the slot-insulating means because of centrifugal forces is very reliably avoided.