Leucocyte populations from rainbow trout subjected to experimental viral haemorrhagic septicaemia virus (VHSV) infections under in vivo and in vitro conditions were analysed by flow cytometry. Quantitative analysis shows that only a low percentage of the leucocytes support viral replication. Lymphocytes, compared with monocytes granulocytes and macrophages, are the least susceptible sub-population. A decrease in the amount of the phagocytic activity was observed after the cells were infected with VHSV. Obvious modifications were observed through dot plot profiles in the composition of the cell sub-populations and in the cell morphology. In addition to a direct effect of VHSV on the macrophages, a systemic effect is produced. This could also be related to the stress induced by the experimental infection process.