Background: Use of enrofloxacin in trout farms is reported, especially for the treatment of yersiniosis, albeit various dosing regimens have been used. Therefore, optimal doses should be investigated. Methods: Five groups of 15 fish were challenged with Y. ruckeri. Two days later, three groups received feed containing enrofloxacin (ENR) at 1, 2.5 and 5 mg/kg fish respectively, during 7 days; one group received a single intraperitoneal injection of ENR at 10 mg/kg; and one group was left untreated. On day 15, surviving fish were euthanized. Results: All fish survived in the group treated by injection, compared to 53%, 60% and 40% of the fish treated with 1, 2.5 and 5 mg/kg oral ENR, respectively, and 53% in the infected untreated group. Conclusion: A single intraperitoneal injection of ENR at 10 mg/kg seems more relevant than repeated oral administrations. The ENR oral doses used in trout farms should be revised.