A turbocharger comprising: a compressor wheel mounted within a compressor housing; a turbine wheel mounted within a turbine housing and connected to the compressor wheel by a shaft; the turbine housing defining an exhaust gas inlet connected to a volute that surrounds the turbine wheel, the turbine housing further defining an axial bore through which exhaust gas that has passed through the turbine wheel is discharged from the turbine housing, the volute comprising a divided volute that defines two separate spaces (58a, 58b) respectively receiving two separate streams of exhaust gas from an internal combustion engine, characterized by: the turbine housing defining a pair of concentrically arranged outer and inner annular bypass passages (60a, 60b) surrounding the bore and arranged to allow exhaust gas in the two separate spaces of the volute to bypass the turbine wheel as two separate bypass streams, the outer bypass passage concentrically surrounding the inner bypass passage; and an annular bypass valve in fluid communication with the bypass passages, the bypass valve comprising a fixed annular valve seat and a rotary annular valve member arranged coaxially with the valve seat relative to an axis, the valve member being disposed against the valve seat and being rotatable about the axis for selectively varying a degree of alignment between respective orifices defined through each of the valve seat and valve member, ranging from no alignment defining a closed condition of the bypass valve, to at least partial alignment defining an open condition of the bypass valve. a compressor wheel mounted within a compressor housing; a turbine wheel mounted within a turbine housing and connected to the compressor wheel by a shaft; the turbine housing defining an exhaust gas inlet connected to a volute that surrounds the turbine wheel, the turbine housing further defining an axial bore through which exhaust gas that has passed through the turbine wheel is discharged from the turbine housing, the volute comprising a divided volute that defines two separate spaces (58a, 58b) respectively receiving two separate streams of exhaust gas from an internal combustion engine, characterized by: the turbine housing defining a pair of concentrically arranged outer and inner annular bypass passages (60a, 60b) surrounding the bore and arranged to allow exhaust gas in the two separate spaces of the volute to bypass the turbine wheel as two separate bypass streams, the outer bypass passage concentrically surrounding the inner bypass passage; and an annular bypass valve in fluid communication with the bypass passages, the bypass valve comprising a fixed annular valve seat and a rotary annular valve member arranged coaxially with the valve seat relative to an axis, the valve member being disposed against the valve seat and being rotatable about the axis for selectively varying a degree of alignment between respective orifices defined through each of the valve seat and valve member, ranging from no alignment defining a closed condition of the bypass valve, to at least partial alignment defining an open condition of the bypass valve. the turbine housing defining a pair of concentrically arranged outer and inner annular bypass passages (60a, 60b) surrounding the bore and arranged to allow exhaust gas in the two separate spaces of the volute to bypass the turbine wheel as two separate bypass streams, the outer bypass passage concentrically surrounding the inner bypass passage; and an annular bypass valve in fluid communication with the bypass passages, the bypass valve comprising a fixed annular valve seat and a rotary annular valve member arranged coaxially with the valve seat relative to an axis, the valve member being disposed against the valve seat and being rotatable about the axis for selectively varying a degree of alignment between respective orifices defined through each of the valve seat and valve member, ranging from no alignment defining a closed condition of the bypass valve, to at least partial alignment defining an open condition of the bypass valve.