A compressor includes a casing defining a generally cylindrical flow passage, a rotor carrying at least one set of rotor blades, at least one set of stator blades, and anti-stall casing treatment. The casing treatment includes an annular recess in the casing for removing low momentum flow adjacent the tips of the rotor blades, and returning the flow to the generally cylindrical flow passage upstream of the point of removal. A plurality of curved guide vanes are located within the annular recess so as to define an annular inlet downstream of the vanes and/or an annular outlet upstream of the vanes. Each guide vane projects radially inwardly from the casing towards a free end which is exposed at or near the mouth of the recess to define a series of curved channels within the recess adjacent the annular inlet and/or the annular outlet.