A turbocharger includes a turbine wheel mounted within a turbine housing and connected to a compressor wheel by a shaft. The turbine housing defines an exhaust gas inlet connected to a divided volute that surrounds the turbine wheel, and an axial bore through which exhaust gas that has passed through the turbine wheel is discharged from the turbine housing. The turbine housing further defines a pair of concentric annular bypass passages surrounding the bore and arranged to allow exhaust gas to bypass the turbine wheel as two separate bypass streams. An annular bypass valve is disposed in the bypass passage. The bypass valve comprises a fixed annular valve seat and a rotary annular valve member arranged coaxially with the valve seat. The valve member is disposed against the valve seat and is rotatable about the axis for selectively varying a degree of alignment between respective orifices in the valve seat and valve member.