Fertilized coho salmonOncorhynchus kisutcheggs were immersed in diluted broth culture of Flavobacterium psychrophilum(1.0×108, 1.0t106, 1.0×104CFU/mL) for 30 min before water-hardening, after water-hardening and at eyed-egg stages. Just after exposure to the bacteria, the eggs were disinfected with povidone-iodine, and then discretely incubated in flowing water. After 7-28 days of incubation, F. psychrophilumwas isolated only from the egg group which had been exposed to 1.0 t 108CFU/mL before water-hardening. No bacteria were detected from the other egg groups exposed to 1.0 t 106or 1.0 t 104CFU/mL as well as the groups exposed to 1.0 t 108 CFU/mL after water-hardening or the group at the eyed-egg stage. The viable counts ofF. psychrophilumin the contents of the infected eggs ranged from 103to 107 CFU/g. Observations on the frozen sections stained by indirect immunofluorescent antibody technique revealed that manyF. psychrophilumcells were located within the infected eggs at the eyed stage. It was concluded that F.psychrophilumentered the eggs during the water-hardening stage.