The properties of the cell surface of Aeromonas salmonicida were studied, with particular emphasis on the additional surface layer (A-layer), found on virulent strains. This was identified by electron microscopy, as having a tetragonal subunit morphology; and by + ejectrophoresis of membrane components as a 51 kdal protein on A strains. A and A- strains, (the latter isolated by growth at elevated temperature), were compared biochemically and their interactions with various cell types investigated. Strains of A. salmonicida possessing the A-layer were shown to be more hydrophobic than those devoid of this protein. The influence of culture age, medium composition and subculture on hydrophobicity were investigated and hydrophobicity related to culture characteristics. No difference in enzyme susceptibility between A+ and A A. salmonicida was found and both phenotypes showed similar tolerance to other environmental conditions. The interactions between A. salmonicida and cells in vitro were studied using adhesion and association assays involving rädiolabelled bacteria, viable count determinations, haemagglutination and chemiluminescence. A+ A. salmonicida were found to adhere to a greater extent than A bacteria to tissue culture cell lines and to isolated fish tissue by non-specific hydrophobic interactions. Adhesion was maximal to a fish epithelial cell line and the effects of various environmental conditions on adhesion were determined. A- bacteria more commonly exhibited haemagglutination which was inhibited by specific sugars. A+ bacteria associated more than A- organisms with mouse peritoneal macrophages and rainbow trout phagocytes when in salts solutions. The effects of opsonization on A. salmonicida were strain dependent. Incubation in a variety of sera resulted in a decrease in A+ surface hydrophobicity, often accompanied by abolition of characteristic autoagglutinability, whereas opsonized A cells became more hydrophobic. These properties directly influenced the chemiluminescence response of trout macrophages.