Yersinia ruckeri is the bacterial agent causing enteric redmouth disease (ERM) in rainbow trout leading to economic losses in intensive aquaculture. There are two main serovars and several minor groups based on O-antigens. The first goal of this thesis was to examine the difference between serotypes of Y. ruckeri in the course of infection in fish by applying in vivo bioluminescent imaging in rainbow trout (Oncorhynchus mykiss) and coho salmon (Oncorhynchus kisutch). In infection trials, the bioluminescent strains were infective, but the bioluminescent signal was not detected in fish infected with bacterial loads of 107 colony forming units per gram of kidney tissue. Skin and scales and the kidney blocked the luminescent signal emitted from the bacteria. The second goal of this thesis was to identify genetic markers which correlate with traditional O-antigen serotyping reactions. Using the sequences of genes which are part of the lipopolysaccharide biosynthetic pathway, oligonucleotide primers were designed to be complimentary to a fragment of wzx, the O-antigen flippase, and to wzy, the O-antigen polymerase, of serotype 1 Y. ruckeri strain RS 11. When these primers were used in polymerase chain reaction, an 1183 bp fragment of wzx and a 755 bp fragment of wzy were seen with DNA from 8 serovar 1 and 9 serovar 1a strains and not from other serovars identified by rabbit anti-sera agglutination. Southern blotting suggested there was little homology between serovar 1 wzx and wzy, and the same genes of the remaining serovars if present.