Summary: Mutants showing oscillatory variation between H-type carrying phase-1 antigen and O-type were isolated from a diphasic derivative of Salmonella typhimurium. The rates of variation from the H-type to O-type were about 10-4/bacterial division and those in the reverse direction 10-3 to 10-4, which are very similar to the rates of variation from the phase-1 to phase-2 of the parent strain and vice versa. Transduction analyses showed that their mutation sites were in H2 (the phase-2 flagellar antigen gene). In O-type bacteria the expression of both the endogenote H1 (the phase-1 flagellar antigen gene) and the exogenote H1 introduced by abortive transduction were repressed. These results indicate that the bacteria are flagellate (H-type) in phase-1 and non-flagellate (O-type) in phase-2, i.e. they are H1-repressor-positive phase-2 non-flagellate mutants (H-O variants). The occurrence of H-O variants strongly supports the hypothesis that there is a special H1-repressor gene in the H2 operon which is concerned with the regulation of H1 expression in phase-2 bacteria.