Protection after intraperitoneal (i.p.) vaccination of yellowtail (Seriola quinqueradiata) against pasteurellosis was studied in a field trial. Yellowtail juveniles captured from the wild or artificially hatched were immunised with an oil-based vaccine against Photobacterium damsela subsp. piscicida, and the fish were observed for 15 weeks after vaccination. Outbreak of pasteurellosis was observed at all five sites (mortality in control group ranged from 7% to 77%), and significant (p<0.01) protection against pasteurellosis relative to non-vaccinated control groups was observed at all sites. The vaccinated fish showed an increased level of agglutinating antibodies against Ph. damsela subsp. piscicida with a peak around 3-4 weeks post vaccination, increased phagocytic activity and increased production of superoxide anions in isolated leucocytes compared to controls, both assessed at 36 and 66 days post vaccination. Transient reduction in fish weight was observed in vaccinated groups until 10 weeks after vaccination; however at 15 weeks, the weight of the vaccinated group was significantly higher than that of the control group. This coincided with the development of side-effect scores at the injection site that had started to wane by 10 weeks, and the downward trend continued up to the last collection time (41 weeks), although with some variation between sites. The study shows that the tested vaccine protects against pasteurellosis in yellowtail under field conditions and that it is safe for use in the target species.