Experiments were performed to determine forced convection heat-transfer coefficients and friction factors for flow in a corrugated duct. The corrugation angle was 30 deg and the interwall spacing was equal to the corrugation height. The Reynolds number, based on the duct hydraulic diameter, ranged from 1500 to 25,000, and the Prandtl number ranged from 4 to 8 (water). Flow visualization, using the oil-lampblack technique, revealed a highly complex flow pattern, including large zones of recirculation adjacent to the rearward-facing corrugation facets. Nusselt numbers in the periodic fully developed regime, when correlated, resulted in a Reynolds-number dependence of Re0.614 and a Prandtl-number dependence of Pr0.34 . The enhancement of heat transfer as compared to a conventional parallel-plate channel was about a factor of 2.5. Friction factors obtained from measured axial pressure distributions were virtually independent of the Reynolds number and equal to 0.57, a value appreciably greater than that for unidirectional duct flows.