A food-borne outbreak of Escherichia coli 0157 occurred at a day nursery in Fukuoka city in June 2002. The causative food was asazuke (salted cucumber). The cause of O157 contamination of asazuke was not clarified by epidemiological investigations . Therefore, laboratory inoculation test using the causative 0157 strain on asazuke during the production process were performed. Additionally, inoculation test with 0157 strains into marketed asazuke products, marketed nukazuke products and homemade shiomomi were undertaken. Results were as follows; (1) O157 organisms rapidly increased when they were inoculated during the initial or early (less than 12 hr) production process of asazuke. No increase, however, was observed following a later (more than 24 hr) inoculation process. (2) O157 organisms inoculated on marketed asazuke and nukazuke did not increase, probably because of large populations of other bacteria. From the above results, the cause of the outbreak was suspected to be O157 contamination and increase which occurred during the initial or early production process of asazuke when other bacteria were low in number.