A luminescence-based detection system was developed to study changes in the survival and activity of cells following release from moribund and dead fish. <I>A.salmonicida</I> was chromosomally marked with the genes encoding bacterial luciferase, originally isolated from <I>Vibrio harveyi</I>. Characterisation of the growth and luminescence of the <I>lux</I>-marked strain demonstrated that light was directly proportional to cell biomass concentration during logarithmic growth. The survival of <I>lux</I>-marked and wild type <I>A.salmonicida</I> strains was investigated in sterile sea water at 4°C. The number of culturable cells declined rapidly, but the total number of cells remained relatively constant, suggesting <I>A. salmonicida</I> entered a nonculturable state. The survival of <I>lux</I>-marked <I>A. salmonicida</I> did not significantly differ from that of the wild type strain. A small number of cells remained culturable throughout starvation experiments and luminometry confirmed that the <I>lux</I>-marked cells were metabolically active, possibly surviving by cryptic growth. The viability of putative dormant cells could not be established since these cells could not be reactivated following the addition of a range of substrates. The <I>lux</I>-marked <I>A.salmonicida</I> strain was pathogenic only when injected at high doses. This poor virulence was probably due to loss of the proteinaceous A-layer which is responsible for hydrophobic cell interactions and cell defence against lytic agents. This prevented further studies aimed at determining the virulence of nonculturable cells using this strain. Preliminary experiments indicated the potential of the <I>lux</I>-marked system for studying vertical transmission of <I>A. salmonicida</I>. The main sites for attachment of the <I>lux</I>-marked strain were the gill and skin/mucus regions. Identical results were obtained using a wild type <I>virulent A. salmonicida</I> strain, but significantly higher numbers of cells were recovered from fish tissue.