Abstract: In a single-flow steam turbine, the inlet casing is designed to comprise two intertwined spiral casings (1, 2). These spirals have concentrically arranged annular openings (1',2') which face the inlet to the blading and extend over 360.degree. of the circumference. The spirals can be shut off and/or throttled, allowing infinitely variable partial admission to the reaction admission (sic) (13, 14, 15). The spirals (2) dimensioned for the smaller flow and their annular opening (2') is arranged on the rotor side in the radial direction. The first row of blading supplied from the annular openings (1', 2') is an after the (sic) action control wheel (13). The radially inner boundary wall of the spiral dimensioned for the small flow is arranged in the plane of the balance piston.