EN
Rainbow trout weighing from 40 to 100 g suspected to be afflicted with redmouth disease were tested in the period from April to November, 1988. Suspected bacteria, i.e. Y. ruckeri were isolated from internal organs (liver, kidneys, intestines) and blood. On the basis of biochemical examinations using 30 different tests 27 strains of Y. ruckeri were determined. They fermented sorbitol, mannitol, mannose, maltose and also other substrates. Taking into account biochemical properties of Y. ruckeri the bacterium was classified as belonning to the phenotype 2. Its pathogenictity was confirmed on the fry of rainbow trout weighing approx. 70 g.