Chinook salmon Oncorhynchus tshawytscha were experimentally infected with Aeromonas salmonicida to determine the dependence of initiation and spread of a furunculosis epizootic on host density. Groups of 30 and 60 recipient Chinook salmon (1.7 +/- 0.14 g) were held in various volumes of water at a wide range of densities (15.52, 7.76, 1.23, 0.61, 0.32, 0.15, and 0.047 g fish/L) for 23-33 d, and each were exposed to a single infectious individual by cohabitation. Significant relationships (r(2) = 0.55) were found between host density (g fish/L) and survival from furunculosis. No pathogen transmission occurred at the lowest density. Acute and chronic patterns of furunculosis were obtained by adjusting densities. It took longer to observe disease-specific mortality among recipient fish at lower densities compared with higher densities even when donors were similarly infectious. The results have important implications for stocking salmonids regarding natural route of transmission and in understanding the pattern of the density effect on furunculosis epizootics in Chinook salmon.