Anorexia is a common sign of bacterial and viral diseases and is thought to be a negative consequence of the disease process. However, this inappetence may be an active portion of the host defence system. We tested this idea by withdrawing food for 32 days from juvenile chinook salmon (Oncorhynchus tshawytscha) during an Aeromonas salmonicida epizootic, induced by cohabitation. Disease-specific mortality was low (5.0% and 12.5% in fed and fasted groups, respectively); there was no mortality in uninfected control fish. While only very few fish had detectable A. salmonicida in the kidney, at the termination of the experiment, an average of 18.5% and 65.0% of the fish in fed and fasted groups, respectively, had this bacterium in or on mucus, but these mean values were not statistically different because of high variation between replicates. Feed intake was measured by X-radiography at days 16 (fed groups) and 32 (all groups). Feed intake as well as growth were unaffected by exposure to bacteria. However, food consumption was greater when fasted fish exposed to A. salmonicida were offered a meal than in those infected individuals that had been eating. This result may be relevant for application of medicated diet, as it seems possible that fasting of sick fish before administration of medicated ration could increase the probability that sick individuals would also eat. (C) 2003 Elsevier Science B.V. All rig his reserved.