Abstract The genus Bordetella contains four species: two are non-motile, the human pathogens B. pertussis and B. parapertussis; and two are motile, the broad host-range mammalian pathogen B. bronchiseptica, and the avian pathogen B. avium. The motility of the latter two species is due to peritrichous flagella. Here we show that strains of all four species contain DNA sequences homologous to flagellin genes. Two types of gene probe were hybridised to Bordetella chromosomal DNA in Southern blots: the structural gene for H1 flagellin of Salmonella typhimurium and an oligonucleotide derived from the conserved N-terminal amino acid sequences of various flagellin proteins. ClaI-digested DNA from all four Bordetella species hybridised with both probes in Southern blots, although each species gave a characteristic pattern of hybridisation. This indicates that the non-motile B. pertussis and B. parapertussis species contain non-expressed flagellin genes.