Abstract
Ribotyping and plasmid profiling were carried out on 299 Danish Flavobacterium psychrophilum isolates from farmed rainbow trout, Oncorhynchus mykiss (Walbaum). The isolates had been characterized biochemically and serologically in another study. The isolates were very homogeneous, 254 isolates had the same ribotype A (restriction enzyme Eco RI) and 284 isolates harboured one 3.3 kb plasmid. Seventy‐five per cent of the F. psychrophilum isolates had ribotype A, one 3.3 kb plasmid and belonged to either serotype Th or Fd. Virulence studies with representatives of the dominant groups classified such isolates as virulent, and an extra small or large plasmid did not change the virulence level. A relationship between the serotypes Fd and Th, certain ribotypes, and virulence was found. The isolates belonging to serotype FpT and to ribotype B were less virulent. Only a few isolates with other ribotypes were found and these were also less virulent. Isolates that did not have the 3.3 kb plasmid were also less virulent. The virulent isolates all harboured the 3.3 kb plasmid; however, there was no clear correlation between virulence and plasmid content as a 3.3 kb plasmid was also found in less virulent isolates.