Aims: The aims of this study were to investigate the effect of ozone and/or negative air ions (NAI) on the viability of bacteria. Methods and Results: Dilute cell suspensions of Pseudomonas fluorescens , Erwinia carotovora pv. carotovora and Escherichia coli were inoculated onto agar and subsequently exposed to ozone and/or NAI. Ozone concentration was maintained at 100 +/- 5 nl l(-1) and NAI at 10(6) ml(-1) . When exposed to a combination of ozone and NAI, viability among all three bacterial species decreased more rapidly when they were inoculated onto potato dextrose agar (PDA) than onto nutrient agar (NA). A subsequent test examined the effect of ozone and NAI alone or in combination on the bacteria inoculated onto PDA only. Treatment with NAI alone had no killing effect on any of the bacterial species. However, a strong interaction between ozone and NAI was observed. Pseudomonas fluorescens was most susceptible to the combined treatment. Cell viability was reduced to 0.7% after 6 h, while 76% of the cells remained viable when exposed to ozone alone. Viability of Erwinia carotovora pv. carotovora was reduced to 4% after 6 h in the combined treatment compared with 69% when exposed to ozone alone. Escherichia coli was relatively more resistant to the combined treatment; viability was reduced to 40% after 11 h compared with 70% in the ozone alone treatment. Conclusions: A strong synergism between ozone and NAI on bacterial cell death was found, but the degree of this effect varied depending on bacterial species. Significance and Impact of the Study: The synergism of ozone with NAI may provide an effective method of reducing food-borne disease and decay of fresh produce.