Campylobacter jejuni 81116 is able to switch flagellum formation on and off. To study the expression of flagellin, the main component of flagella, an expression library of C. jejuni DNA was constructed in lambda gt11. Screening of this library with a flagellin-specific antiserum resulted in a clone producing a beta-galactosidase-flagellin fusion protein; it contained a DNA insert of 850 base pairs and coded for about 15 kilodaltons of the flagellin, corresponding to 410 base pairs of the flagellin gene. To study the regulation of the on-and-off switch of flagellum production, a nonmotile variant was isolated from semisolid medium. Western blots (immunoblots) showed the absence of flagellin in the nonmotile form. Southern blots of digested DNA of both motile, flagellate bacteria and nonmotile, aflagellate bacteria were identical, while Northern (RNA) blot analysis showed the absence of flagellin mRNA in the aflagellate form. Thus, it is concluded that reversible flagellin expression is regulated at the transcriptional level. Southern blots suggest that more than one flagellin gene is present. The structure and function of campylobacter flagellin can now be further investigated at the DNA level.