Measurements of the flow field and thermal field are reported for a flow in a periodically ribbed duct al two Reynolds number, 3400 and 24 000. At the lower Reynolds number, the incoming flow is transitional, while at Re = 24 000, the incoming flow is fully turbulent. Thus, the reported measurements highlight the differences in the development of the surface heat transfer of a flow undergoing transition (Re = 3400) with that of a fully turbulent flow (Re = 24 000) in a ribbed duct. At Re = 3400, the heat transfer in the first interrib module, due to the transitional nature of the flow, is lower than the hear transfer in the second interrib module. For the Fully turbulent flow, the heat transfer profile in successive interrib modules decays to the periodic state. Associated velocity measurements confirm the transitional behavior al Re = 3400, with nearly a twofold increase in the normalized streamwise velocity fluctuations past the first interrib module, where transition is completed. At Re = 24 000, no such dramatic increases of streamwise turbulence levels is noted. The detailed set of measurements provides a complete picture of the transitional and fully turbulent flow and heat transfer in a ribbed duct.