Abstract: A screw compressor of the kind incorporating at least one male rotor and one female rotor, with helically extending lands and grooves. The female rotor (1) is injection moulded from a plastics material having a modulus of elasticity of at most 25,000 N/mm.sup.2. The lands (3) of the female rotor have a thickness (a) which is so adapted to the modulus of elasticity of the plastics material as to allow the lands to be deflected resiliently when clashing with the other rotor (2), as a result of dimensional deviations of the order likely to occur during hardening or curing of the plastics material, or at prevailing temperature variations, but not as a result of the pressure exerted by the working fluid. Due to the injection moulding the surface smoothness is fine enough to make subsequent finishing unnecessary and damages caused by lack of machining are avoided because of the resilient deflecting of the lands of the female rotor when clashing with the other rotor.