The orientation of collagen fibers in the growth plate and contiguous structures of a growing long bone was demonstrated by polarized light microscopy. Five major groups of collagen fibers were demonstrated: transphyseal (longitudinal), perichondrial-periosteal (longitudinal), epiphyseal (radial), perichondrial ring (circumferential), and metaphyseal bone (circumferential). Transphyseal collagen fibers extend from spicules of calcified cartilage in the metaphysis across the growth plate and into the epiphyseal cartilage and secondary ossification center. The transphyseal fibers interdigitate with radially oriented epiphyseal fibers which lie between the secondary ossification center and the zone of resting cells. A radial columnar alignment of cells, similar to the longitudinal cell columns of the growth plate, was correlated with the radial epiphyseal collagen fibers. Collagen fibers that are longitudinally oriented predominate in the perichondrium-periosteum. In the primary spongiosa, bone collagen is oriented obliquely and circumferentially on the longitudinal septa of calcified cartilage. A marked abundance of circumferentially oriented collagen fibers is seen within the perichondrial groove and in the perichondrium-periosteum directly over the groove. The perichondrial rings is the largest and most prominent of these circumferential groups.