A screw compressor having a valve for varying a volume ratio of the screw compressor, the screw compressor comprising: a compressor housing comprising: a screw rotor bore; a suction port in fluid communication with a first end of the rotor bore; and a discharge chamber in fluid communication with a second end of the rotor bore, the discharge chamber having a discharge chamber pressure; and intermeshing male and female screw rotors (26a,26b) disposed within the screw rotor bore, the intermeshing male and female screw rotors having lobes defining a compression pocket with the rotor bore, the compression pocket having a compression pocket pressure; characterised by a valve body disposed along the screw rotor bore between the intermeshing male and female screw rotors, the valve body comprising: a duct extending into the valve body and including an open end thereof in fluid communication with the discharge chamber and the discharge chamber pressure; an auxiliary port extending from the rotor bore to the duct and providing fluid communication therebetween for communicating the compression pocket pressure to the duct; and a reed valve disposed in the duct for regulating fluid flow between the compression pocket and the duct, the reed valve being operable via a pressure differential between the compression pocket pressure and the discharge chamber pressure. a compressor housing comprising: a screw rotor bore; a suction port in fluid communication with a first end of the rotor bore; and a discharge chamber in fluid communication with a second end of the rotor bore, the discharge chamber having a discharge chamber pressure; and a screw rotor bore; a suction port in fluid communication with a first end of the rotor bore; and a discharge chamber in fluid communication with a second end of the rotor bore, the discharge chamber having a discharge chamber pressure; and intermeshing male and female screw rotors (26a,26b) disposed within the screw rotor bore, the intermeshing male and female screw rotors having lobes defining a compression pocket with the rotor bore, the compression pocket having a compression pocket pressure; characterised by a valve body disposed along the screw rotor bore between the intermeshing male and female screw rotors, the valve body comprising: a duct extending into the valve body and including an open end thereof in fluid communication with the discharge chamber and the discharge chamber pressure; an auxiliary port extending from the rotor bore to the duct and providing fluid communication therebetween for communicating the compression pocket pressure to the duct; and a reed valve disposed in the duct for regulating fluid flow between the compression pocket and the duct, the reed valve being operable via a pressure differential between the compression pocket pressure and the discharge chamber pressure. a duct extending into the valve body and including an open end thereof in fluid communication with the discharge chamber and the discharge chamber pressure; an auxiliary port extending from the rotor bore to the duct and providing fluid communication therebetween for communicating the compression pocket pressure to the duct; and a reed valve disposed in the duct for regulating fluid flow between the compression pocket and the duct, the reed valve being operable via a pressure differential between the compression pocket pressure and the discharge chamber pressure.