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 turbine housing defining an annular bypass passage surrounding the bore and arranged to allow exhaust gas to bypass the turbine wheel; and an annular bypass valve disposed in the bypass passage, 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 defining a plurality of first orifices therethrough and the valve seat defining a plurality of second orifices therethrough, with each first orifice having a corresponding second orifice, 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, wherein the valve member is a generally flat annular disk having the first orifices circumferentially spaced apart about a circumference of the valve member and the valve seat is a generally flat annular disk having the second orifices circumferentially spaced apart about a circumference of the valve seat, there being sufficient circumferential distance between adjacent second orifices to accommodate the first orifices therebetween in the closed condition of the bypass valve, and wherein the first orifices have a first circumferential spacing and the second orifices have a second circumferential spacing, wherein the first circumferential spacing is different to the second circumferential spacing. 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 turbine housing defining an annular bypass passage surrounding the bore and arranged to allow exhaust gas to bypass the turbine wheel; and an annular bypass valve disposed in the bypass passage, 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 defining a plurality of first orifices therethrough and the valve seat defining a plurality of second orifices therethrough, with each first orifice having a corresponding second orifice, 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, wherein the valve member is a generally flat annular disk having the first orifices circumferentially spaced apart about a circumference of the valve member and the valve seat is a generally flat annular disk having the second orifices circumferentially spaced apart about a circumference of the valve seat, there being sufficient circumferential distance between adjacent second orifices to accommodate the first orifices therebetween in the closed condition of the bypass valve, and wherein the first orifices have a first circumferential spacing and the second orifices have a second circumferential spacing, wherein the first circumferential spacing is different to the second circumferential spacing.