To clarify the factors for occurrence of extended-spectrum β-lactamase (ESBL)-producing Escherichia coli in broilers, two flocks (1 day of age) fed a diet with or without antibiotics were kept in a broiler house sanitized with disinfectants. ESBL-producing E. coli, however, was detected at a concentration of over 10(6) CFU/g of feces at 9 days of age to 49 days of age in both broiler flocks. Therefore, this indicated that the antibiotics other than cephalosporins used in this study had no effect due to co-selection on the numbers of ESBL-producing E. coli in broiler feces during this period. When a flock was kept with diet containing antibiotics for 49 days in a laboratory animal room, no ESBL-producing E. coli was detected in the flock. These results suggest that the occurrence of ESBL-producing E. coli may not be related to feeding with antibiotics and that the contamination of broiler houses with ESBL-producing E. coli might be an important factor.