Colonisation and transmission of Aeromonas salmonicida in Atlantic salmon was investigated using wild-type and lux-marked strains of A. salmonicida. An initial intra-peritoneal (i.p.) challenge showed that lux-marked cells were virulent only when injected at concentrations ≥109 cfu ml−1 and significantly less infective than wild-type MT463. The low virulence of A. salmonicida MT463 luxAB was probably due to loss of the proteinaceous A-layer, which is an important virulence factor involved in both intra- and inter-cellular A. salmonicida interactions. During the i.p. challenge, all fish were held in one tank enabling assessment of transfer of lux-marked A. salmonicida between fish. Lux-marked cells shed from moribund and dead fish survived in the water column and cross-infected cohabitant fish. Cross-infection by A. salmonicida MT463 luxAB was investigated further by carrying out a cohabitation challenge. Lux-marked cells were recovered in low numbers from gill tissue and skin/mucus of cohabitant fish. Poor adhesion of cells may be due to loss of the A-layer protein. During a second cohabitation challenge using A-layer+ and virulent wild-type strain MT432, between 102 and 107 cells g−1 of fish gill tissue or skin/mucus were isolated. This result confirmed the preliminary observations obtained using lux-marked A. salmonicida MT463 and suggested that the gill and skin/mucus regions of fish were the main sites for attachment of A. salmonicida. None of the A. salmonicida strains was recovered from fish intestine samples during cohabitation challenges.