An investigation was made at a Mach number of 3.12 to determine the effects of heat transfer on boundary-layer transition. Data were obtained for a cone cylinder and a parabolic-nosed cylinder at Reynolds numbers up to 12,000,000 based on body length. The results show that cooling the cone-cylinder model to a wall-to-free-stream static-temperature ratio of approximately 1.4 increased the transition Reynolds number from a value of about 2,000,000 at equilibrium to 10,600,000. For temperature ratios less than 1.4, the boundary-layer flow was laminar over the entire model. The rapid increase of transition Reynolds number with small reductions in temeprature ratio near 1.4 indicated that temperature ratios slight lower may result in a laminar boundary layer for very high Reynolds numbers. For the parabolic-nosed body, the transition Reynolds number was about twice that of the cone-cylinder model over the temperature range investigated.