Diagonal ventilating fan that comprises: a fan housing; a fan wheel which is rotatable about a rotation axis and to which an electric motor is assigned in order to drive it, the fan wheel extending in the direction of the rotation axis between an air inlet side and an air outlet side of the diagonal ventilating fan and serving, as the fan wheel rotates, to convey air from the air inlet side to the air outlet side in a main conveying direction; fan blades which are provided on the outside of the fan wheel and which have a front side and a rear side in relation to the rotation direction of the fan wheel during operation, a negative-pressure zone arising on the fan wheel during operation in the region of the rear side of each blade; characterised in that an auxiliary cooling-air blower is provided which is configured to transport cooling air on the inside of the fan wheel from the air outlet side towards the air inlet side, and by means of said air flow, to cool the electric motor of the diagonal ventilating fan; at least one fluid connection (114; 118; 130) from the outlet of the auxiliary cooling-air blower to at least one of the negative-pressure zones is provided in order to assist a flow (126; 126') of cooling air for the electric motor, which flow occurs from the air outlet side of the diagonal ventilating fan towards the air inlet side thereof, and the fan wheel comprises, on the air outlet side of the diagonal ventilating fan, a first annular portion that interacts with an opposite second annular portion of a stationary part of the diagonal ventilating fan, and air-directing elements being provided on this second portion, which air-directing elements act, during operation, as a brake for the air that is entrained by a rotation of the first portion. a fan housing; a fan wheel which is rotatable about a rotation axis and to which an electric motor is assigned in order to drive it, the fan wheel extending in the direction of the rotation axis between an air inlet side and an air outlet side of the diagonal ventilating fan and serving, as the fan wheel rotates, to convey air from the air inlet side to the air outlet side in a main conveying direction; fan blades which are provided on the outside of the fan wheel and which have a front side and a rear side in relation to the rotation direction of the fan wheel during operation, a negative-pressure zone arising on the fan wheel during operation in the region of the rear side of each blade; an auxiliary cooling-air blower is provided which is configured to transport cooling air on the inside of the fan wheel from the air outlet side towards the air inlet side, and by means of said air flow, to cool the electric motor of the diagonal ventilating fan; at least one fluid connection (114; 118; 130) from the outlet of the auxiliary cooling-air blower to at least one of the negative-pressure zones is provided in order to assist a flow (126; 126') of cooling air for the electric motor, which flow occurs from the air outlet side of the diagonal ventilating fan towards the air inlet side thereof, and the fan wheel comprises, on the air outlet side of the diagonal ventilating fan, a first annular portion that interacts with an opposite second annular portion of a stationary part of the diagonal ventilating fan, and air-directing elements being provided on this second portion, which air-directing elements act, during operation, as a brake for the air that is entrained by a rotation of the first portion.