The effects of restricted geometries on the swimming behavior of Escherichia coli were examined. Videomicroscopy and image analysis techniques were used to measure single-cell motility parameters (cell velocity, tumbling probability, and turn angle) in microfabricated 2-D channels ranging from 2 to 20 mu m in height. Macroscopic motility coefficients were calculated from the single-cell data and compared to literature values. These observations indicate that single-cell parameters remain nearly constant in confined spaces in which channel heights approach the size of the bacterium. The nearly constant swimming behavior of individual cells in confined geometries indicates that the macroscopic motility coefficient describing population behavior will be greater than the unrestricted value by a factor proportional to the dimensionality of the system.