Abstract: A clamp assembly for adjustably attaching a bicycle or exercise device seat having seat support rails to a seat support post includes two spaced pairs of inner and outer cheekpieces which have parallel abutting surfaces inclined at an angle to the vertical, each of the cheekpiece pairs defining a seat rail receiving groove therebetween. Threaded fasteners attach the cheekpieces to each other and to a clamp support structure such that the cheekpieces slide relative to each other along a plane between the parallel abutting surfaces to open or close the grooves. The cheekpieces may also be pivotally mounted on the clamp support structure for simultaneous adjustment of the angle of inclination of the grooves and seat rails clamped therein merely by adjustment of the same threaded fasteners.