A TEM study on proeutectoid ${alpha}$ reaction at sub-eutectoid temperatures has been made in hypoeutectoid Ti-3.gw/o Co and Ti-5.2w/o Fe alloys. Widmanstatten ${alpha}$ plates were formed in both alloys within the ${eta}$ matrix with some modification of degenerate forms. These degenerate plates were formed by the sympathetic nucleation of ${alpha}$ plates at ${alpha}:{eta}$ interphase boundaries. Three types of sympathetic nucleation, i.e., edge-to-edge, face-to-edge, face-to-face, were found in both alloys. The edge-to-edge sympathetically nucleated crystals formed a low-angle boundary between two crystals. The ${alpha}:{eta}$ interphase boundaries were found to be partially coherent interfaces which consist of regularly spaced misfit dislocations. The growth of these interphase boundaries were accomplished by the lateral movement of growth ledges. The intersection points of two ${alpha}$ plates or the low angle boundaries which had formed by edge-to-edge sympathetic nucleation played a role as the potential sources of growth ledges during the growth of plate. The interfacial structures and the spatial morphologies of the degenerate proeutectoid ${alpha}$ plates would be expected to influence the nucleation and growth of the succeeding eutectoid decomposition process.