An auxiliary heater suitable for mobile applications and provided with a burner and a cup-shaped heat exchanger (10) which has a hollow cylindrical wall section (26, 48) with a first end area (16) facing the burner, a bottom section (24, 46) facing away from the burner, a heat transfer medium inlet (18), and a heat transfer medium outlet (20), by which, in operation of the auxiliary heater, a fluid heat transfer medium flows through the bottom section (24, 46) and the hollow cylindrical wall section (26, 48). In order to devise an auxiliary heater with has higher efficiency than known auxiliary heaters of the same size, both the heat transfer medium inlet (18) and heat transfer medium outlet (20) located on the first end area (16) of the heat exchanger (10).