A series of experiments was carried out with infectious hematopoietic necrosis virus (IHNV; 193-110 isolate) in rainbow trout Oncorhynchus mykiss (weight, similar to 1.2 g) to determine the duration of the patent period and the timing of onset of the infectious periods. We first attempted to transmit IHNV to recipient fish from infected rainbow trout 2-3 d after they had been exposed. No infection transfer occurred despite high titers (10(4.79) to 10(4.91) plaque-forming units 5-8 d postexposure (dpe). To determine the number of secondary cases produced by one infectious individual, we exposed approximately 50 rainbow trout (weight, similar to 1.5 g) in each of seven replicate tanks to a donor fish that had been infected with virus by bath exposure 3 d earlier. The prevalence of infection in recipient fish rose from 0.84% at 2 dpe to 7.9% at 6 dpe. Maximum incidence (22 cases) occurred between 2 and 4 dpe. No disease-specific mortalities occurred in recipient fish during the experiment. The titer of virus in both recipient and donor fish increased from 2 to 4 dpe. There was a positive correlation between the level of infection among donors and prevalence values among recipient fish (r(2) = 0.60). The level of challenge by one infectious fish under the conditions provided was enough for infection transfer from sick cohabitant to susceptible fish but was not enough for initiation of a full-scale epizootic among recipients.