An electron microscopic study of osteons in the cortical bone of the mid-femoral shaft of six adult and four immature mongrel dogs is reported. Embedding was done with a methacrylate for the preliminary study of Haversian canals and with a plastic resin (Epon 812) for the study of intracellular detail. The anatomical features and relationships of the vascular and neural components, the cells (undifferentiated mesenchymal cells, osteoblasts, and osteocytes), and matrix (unmineralized and mineralized) are described. The physiological significance of the morphological findings is discussed on the basis of available evidence.