Abstract: A ball spline bearing assembly includes an outer, hollow cylinder, a guide shaft which extends through the cylinder and a plurality of balls interposed between the splined, opposite surfaces of the cylinder and the shaft. The cylinder is provided with an end member formed with a required number of connection passages at each end, so that two adjacent guide passages defined between the splined, opposite surfaces of the cylinder and the shaft are connected by the connection passage at each end thereby defining an endless path along which the balls can roll indefinitely. A tubular-shaped ball retainer is interposed between the shaft and the cylinder and is formed with a plurality of elongated windows. The ball retainer serves to keep the balls in rolling contact with both the cylinder and the shaft in a set of alternate straight guide passages; however in the other set of alternate straight guide passages they are in rolling contact with either the cylinder or the shaft.