This paper deals with the calibration of bed resistance to flow in a straight open channel contained within a laboratory flume, The longitudinal slope of the flume was able to be varied, and so the flow could be subcritical or supercritical. The flow was in the transitional turbulent region, with Reynolds numbers in the range 3 x 10(4) to 2 x 10(5). The calibration demonstrates experimentally a prediction linking the value of Manning＇s n with longitudinal slope in this region, and indicates a useful approach to define this relationship. This is of benefit for laboratory work in the smooth or transitional turbulent region, particularly if it is wished to employ Manning＇s formula because of its extensive use in river engineering practice.