Abstract: The casing 24 of an axial flow compressor 12 is provided with a plurality of axially extending circumferentially spaced slots 30 in its internal cylindrical surface 32 adjacent the tips of at least one row of blades 26. A benefit in both surge margin improvement and a reduction in efficiency deficit may be achieved by positioning the leading edge of the slot 30 such that it leads the leading edge of the blade 26 by an amount termed the overhang or by reducing the closed to open ratio of the slots 30. A further reduction in efficiency deficit may be achieved by combining the overhang which individually gave the best surge margin improvement with a slot 30 closed to open ratio somewhat higher than the value which individually gave the best surge margin improvement.