Amplification of multiple P multocida genomic DNA fragments by outwardly-directed primers based on the repetitive extragenic palindromic (REP) consensus sequence, generated complex profiles in a PCR-based fingerprinting method known as REP-PCR. Polymorphisms within REP-PCR profiles were used to characterise 38 isolates of P multocida. The high degree of homogeneity observed among haemorrhagic septicaemia (HS) strains of serotype B and E provided evidence of a disease-associated REP profile that may serve as a novel method for the identification of Hs strains regardless of serotype. REP-PCR profiles of other P multocida serotypes were highly variable, illustrating the potential of this technique for the molecular fingerprinting of fowl cholera or atrophic rhinitis isolates. A specific amplified REP fragment was isolated and used to probe membrane-bound digested P multocida genomic DNA. Hybridisation patterns not only distinguished HS-causing isolates from non-HS P multocida, but also demonstrated a degree of relatedness between HS and HS-like strains.