The results of an experimental investigation of the effects of a flexible boundary on pressure drop in turbulent flow in a pipe are reported. A test section was made by casting a highly compliant, elastic gel between two concentric pipes. The inner pipe was then removed leaving a flexible boundary for flow studies. Fluid was caused to flow through the test section and pressure drop measurements were recorded at Reynolds numbers ranging from 1800 to 15,200. The pressure drop across the flexible boundary was found to be greater than that encountered in a similar section with a solid boundary. Below Re 13,500, this increase can be explained either in terms of effective wall roughness or by an effect of the flexible boundary on turbulence. Above Re 13,500, the data indicate a pressure drop increase due to the effects of the flexible boundary. (Author).