The bactericidal activity of gaseous ozone was investigated using a commercial ozone generator. Five species of fish bacteria, Pseudomonas putida, Shewanella putrefaciens, Brochothrix thermosphacta, Enterobacter sp. and Lactobacillus plantarum, were inoculated on agar surfaces and exposed to different ozonation times in a gas chamber. Results showed ozone in relatively low concentrations (< 0.27 x 10(-3) g l-1) was an effective bactericide of vegetative cells of the five fish bacteria. The age of the cell culture was shown to influence the cell response following exposure. Survival rate was not linearly related to ozonation time, but exhibited biphasic death over an extended period. Similar bactericidal effects were observed on fish skin treated with ozone daily in the laboratory, with decreases of 1.0 log cfu cm-2 for the micro-organisms studied. Whole fish treated daily in the laboratory using a commercial ozone generator showed improved scores for sensory analyses compared with the controls. The results were statistically significant. Fish treated on board ships were also analysed for microbiological and sensory changes. Controls were obtained from a similar vessel without the ozone facility in the hold. Similar trends to those recorded in the laboratory for the microbiological and sensory results on ozonated fish were observed.