The susceptibility of chicks to colonization with Escherichia coli O157: H7 and other verotoxin-producing serotypes was studied. The E. coli colonized the caeca of both broilers and layers after oral administration of the challenge. The extent of colonization was dependent on challenge dose, age of chicks at the time of challenge, breed of chicks and length of time after exposure. A rapid increase in caecal colonization and a decrease in crop colonization was evident during the first few hours after challenge, with a maximum in the caeca at 6 h. Escherichia coli persisted in caecal contents as well as on caecal walls throughout the course of the experiments (14 d), but at much lower levels compared with the 6 h post-challenge.