A compressor or fan casing is provided with a casing treatment to improve the surge margin of an unshrouded rotor stage. The casing treatment takes the form of an insert (20) let into the casing wall (10) to provide a plurality of re-circulation grooves (22) in the wall circumscribing the path of the rotor blade tips. The insert consists of a plurality of arcuate insert segments (20) made of a carbon fiber reinforced resin. In the preferred construction each arcuate segment (20) is formed with axially extending tangs (28,29) on either side that are received into the correspondingly shaped sides (38,40) of a receiving channel (16) in the casing wall (10). The casing (10) may be split on a diametric plane, thus exposing groove ends (38,40), into which the tangs (28,29) carried by the inserts (20) can be slid before the casing (10) is finally assembled.