A vertebral fixation plate system and a method for its use in a surgical procedure to fuse adjacent spinal vertebrae. A mounting member is attached to the vertebral fixation plate so that the fixation plate can move with respect to the mounting member through a range of available positions. The mounting member can be attached to a spinal interbody fusion device installed in place of a removed spinal disc, thus also loosely fastening the vertebral fixation plate to the fusion device. A fastener can tighten the vertebral fixation plate to the mounting member and establish a location of the vertebral fixation plate with respect to the vertebrae to be fused. Confronting surfaces of the mounting member and the vertebral fixation plate cooperate to keep the vertebral fixation plate in a desired location while it is fastened to the vertebrae being fused.