The near-wall structure of a turbulent boundary layer has been studied using hot-film/wire anemometry in conjunction with a flow visualisation technique using a smoke-wire and a sheet of laser light. The main objective of the study was to investigate the mechanism of drag reduction on a riblet wall reported by Walsh (1980 and 1982) and Walsh and Lindemann (1984) through a comparison of the turbulence structure on a riblet wall with that on a smooth wall .