We isolated Salmonella choleraesuis subsp. choleraesuis serovar. Enteritidis (S. Enteritidis) from a mass of cases which broke out in September, 1991 in Kumamoto City. The isolates were shown to hold plasmids on which a tetracycline (TC)-resistant gene was located. The plasmid, about 9 kb in size, was capable of expressing the gene in Escherichia coli, unstable in S. Enteritidis and Escherichia coli. Thus in order to investigate the creeping prevalence of the TC-resistant plasmid in Kumamoto City and its spreading in other areas, the strains, a total of 41, including 37 isolates from sporadic cases with diarrhea in Kumamoto City, 3 isolates from 3 cases of food poisoning in Kitakyushu, Fukuoka and Chiba, and one strain isolated from egg solution in Kumamoto City, were examined by the hybridization test with this plasmid as a probe, as well as for their drug sensitivity and plasmid profile. As a result, 28 strains of 37 tested from the sporadic cases, the strains from food poisoning in Kitakyushu, Chiba and the one from egg solution, all harbored the same 9 kb TC-resistant plasmid. From the above observations, it was assured that, at the time of investigation, July to September in 1992, the prevalence of S. Enteritidis in Kumamoto City was Predominated by its strain carrying the TC-resistant plasmid. Furthermore, considering the fact that the strain harboring this plasmid was also found in the food poisoning in Kitakyushu and Chiba, it was suggested that spreading of this strain was not restricted to localities and was also associated with Chicken eggs according to its detection from the egg solution.