PCR fragments, 1500-bp, from 15 previously sequenced regions in the Escherichia coli chromosome have been compared by restriction analysis in a large set of wild (ECOR) strains. Prior published observations of segmental clonality are confirmed: each of several sequence types is shared by a number of strains. The rate of recombinational replacement and the average size of the replacements are estimated in a set of closely related strains in which a clonal frame is dotted with occasional stretches of DNA belonging to other clones. A clonal hierarchy is described. Some new comparative sequencing data are presented.