Motility of Escherichia coli K-12 cells was studied in a capillary tube with an inside diameter of 3 mum, which is comparable to the size of the cells’ body. The extreme restriction, imposed by the capillary on the bacterial motility, caused a series of phenomena such as cell aggregation, swimming as a cluster, and break-up of aggregates, which were observable for the first time, and which are reported here. (c) 1997 John Wiley & Sons, Inc.