Abstract: A compressor having either an axial or radial stage adjacent the air inlet includes a circumferential row of slots in the inner casing wall, the slots opening into the casing bore; a rotor having a leading row of compressor blades thereon rotatable within the bore with the tips of the blades adjacent the row of slots; and, the axial centerline of the circumferential row of slots being positioned axially forward of the axial centerline of the blades of the leading blade row with a substantial portion of the axial slot length of each slot being forward of the leading edge of the blades of the leading blade row. In a preferred form of the compressor, the inner casing wall includes a circumferential stationary ring having an inner wall for defining with the inner casing wall the bore in which the rotor rotates, the circumferential row of slots being formed in the ring wall.