Previous experimental studies have found that surface interactions significantly affect the transport of motile bacteria through small tubes, along solid surfaces, and through porous media. However, the role that hydrodynamic forces play in the interactions between solid surfaces and motile bacteria remains unclear. In this study, the swimming speeds of populations of Escherichia coli bacteria were measured near (<10 mu m) and far (>10 mu m) from a flat glass surface at four ranges of orientations to the surface (0 degrees-45 degrees 45 degrees-90 degrees, 90 degrees-135 degrees and 135 degrees-180 degrees). Populations of bacteria close to the surface and moving in the orientation range most perpendicular (0-45 degrees) to the surface experienced the greatest change in the swimming speed when compared to the population in the same orientation range located far from the surface. The decrease in swimming speed experienced by this population was on the same order as that predicted by hydrodynamic models of bacterial swimming near surfaces.