Eighteen strains of Salmonella choleraesuis subsp. choleraesuis serovar Typhi (S. typhi) isolated from blood of patiens in Japan who had visited Indonesia and returned before the onset of typhoid fever were found to possess the H-antigen z66reported by Guinée et al. (1981) but had the naturally occuring H-antigen d. They were not lysed by any of the phages of the Vi phage typing system. After passage through semi-solid medium containing H-z66 antiserum, H-antigens of 11 of 18 cultures with H-z66 phage changed to H-j and those of 2 others to H-d, while the remaining 5 were immobilized. With cultures in the H-d or H-j phages, change of the H-antigens did not occur when they were cultured in semi-solid medium containing homologous H-antiserum. These phase induction experiments as well as colony examination of original cultures suggest that H-z66 phase is unstable and tends to change of H-antigen to H-d or H-j occured without difficulty probably represented a mixed population of cells in the H-z66 and H-d or H-j phases. Since none of 856 isolates from Japan or fromimported cases of typhoid fever from Southeast Asia other than Indonesia exhibited the H-z66 antigen, it was concluded that the focus of typhois fever caused by S. typhi in H–z66 phase was probably in Indonesia.