Biochemical and pathogenicity studies of bacterial isolates from onions with discoloured internal tissues indicated the fluorescent pseudomonadPseudomonas aeruginosa(Schroeter, 1872) Migula, 1900 to be the causal agent and that a site of injury was necessary for infection to take place. It is postulated that in the field a likely mode of infection would be through the necks of onion bulbs which are damaged during or after harvest.