A camshaft phaser comprising a differential bevel gear arrangement to vary the phase relationship of a camshaft to a crankshaft in an internal combustion engine. In the differential gear system, a 45.degree. beveled input gear is mounted parallel to and coaxial with a 45.degree. beveled output gear. One or more 45.degree. beveled spider gears is disposed in meshed relationship with the input and output gears in a gear pattern having a rectangular cross-sectional appearance. Rotation of the input gear causes an opposite rotation of the output gear. The phase relationship between the input and output gears may be varied by varying the position of the spider gear. The input gear and spider gears may be driven via a sprocket in time with the crankshaft in a plurality of arrangements.