This paper describes an inexpensive form of casing treatment which was found to increase both efficiency and stall margin in an axial compressor stage and which could be easily and cheaply retrofitted to existing engines. The method consists of placement of a row of vortex generators on the outer casing of an axial compressor, upstream of the rotor. Design techniques are described. An experimental investigation is also described in which vortex generator tip treatment was applied to a single stage axial compressor. Gains in compressor efficiency and stall margin were observed.