SummaryTheasoAgene of Aeromonas salmonicida is located approximately 7 kb downstream of the A‐layer structural gene, vapA. A 6 kbBamHI fragment containingasoA was cloned and marker‐exchange mutagenesis using a kanamycin‐resistance cassette was performed to generate anasoA mutation in the low‐virulence strain A449L. When analysed by electron microscopy, the mutant A449L‐MB exhibited an altered surface morphology. Strands and blebs of membranous material were observed protruding from the disorganized cell surface. This material was shown to contain lipopolysaccharide and A‐layer subunit protein. The disorganization of the surface of A449L‐IV1B had no apparent effect on virulence when the bacteria were administered to rainbow trout (Oncorhynchus mykiss) by bath Immersion. However, when administered by intraperitoneal injection, the mutant A449L‐MB was found to exhibit significantly increased virulence. The predicted amino acid sequence of AsoA shows homology to a number of polytopic membrane proteins involved in translocation across the cytop