Escherichia coli population dynamics and diversity in rats fed diets differing in their crude fibre content were assessed. Female Wistar rats (n=40) were fed diets containing 1, 4, 18 or 26% crude fibre. Animals were housed in pairs, and one animal was inoculated with a phylogroup B1 strain of E. coli, the other with a phylogroup B2 strain. Natural strain transmission was allowed to occur between the animals in each cage. As expected, the diets had a significant effect on gut dynamics. Mean gut retention times were shorter in animals fed the 18 and 26% crude fibre diets compared with animals on the low-fibre diets. The effect of diet on gastrointestinal dynamics in turn affected E. coli population dynamics and clonal composition. Animals fed the low-fibre diets had higher cell densities than animals fed the high-fibre diets. E. coli populations dominated by phylogroup B2 strains exhibited lower cell densities in animals fed the high-fibre diets compared with cell densities in animals fed the low-fibre diets. Overall, E. coli cell densities declined as gut transit times decreased. Results from this experiment support the results garnered from prospective studies examining the distribution of E. coli from hosts with differing diets, gut morphology and dynamics.