In August 1983 the Danish Trout Culture Research Station investigated a disease problem in a rainbow trout farm in Mid-Jutland. The following clinical symptoms were observed: haemorrhages in the eyes, ulcerations in the mouth region, and haemorrhages in the mouth cavity. Although the haemorrhages in the eyes could be due to a VHS infection, the overall symptoms suggested a bacterial infection. The affected fish weighed on average 150g and the water temperature at the onset was approximately 16°C. The anterior kidney from 4 fish was sampled aseptically for bacteriological examinations. The initial cultures were made on blood agar incubated at 20°C for 48 h. The inoculated plates showed a pure growth of one colony type. The organism was shown to be a Gram-negative rod, fermenting glucose in O/F medium. Diagnosis was made biochemically. The biochemical reactions were very similar to those of Yersinia ruckeri, the redmouth (RM) bacterium. The trout farm was visited again at the end of September 1983, the water temperature was at that time 12°C. Samples of the anterior kidney from five moribund fish were taken. A pure culture, identified as Yersinia ruckeri, was isolated from all 5 fish. The infection did not cause heavy mortality.