Rainbow trout fry were infected by a pathogenic Sp type of infectious pancreatic necrosis virus isolated in France. Fry held at lO'C and infected at different ages showed a decrease of sensitivity with increasing age and ceased to be susceptible to the disease when 20 weeks old. Mortality was delayed at 6'C and lowered or suppressed at 16'C. When fry were moved from 10 to 16'C before infection mortality was also lowered but not when they were moved from 16 to lO'C. Late infection of sihlings kept at different temperatures before infection suggested a relation between the number of degrees x days and the final mortaUty recorded.