We studied the ribotypes, patterns of pulsed-field gel electrophoresis (PFGE) and interspersed repeated sequences (IRS)-PCR of 30 strains of Yersinia ruckeri O1 and 20 strains of Photobacterium damsela subsp. piscicida isolated from apparently unrelated epizootic outbreaks occurring on Italian fish farms between 1993 and 1999. All of the Y. ruckeri O1 strains had similar profiles, as demonstrated by all three typing methods, thus confirming the clonal structure of this species. The strains of P. damsela subsp. piscicida showed similar profiles when tested with ribotyping and PFGE; however, two slightly different profiles were distinguished by IRS-PCR using the primer ERIC2. The two profiles appeared to be related to the geographic origin of the strains, since each of them was specific to isolates from a certain area (i.e. northern or southern Italy). This result suggests that PCR fingerprinting may be a valuable tool in typing fish bacterial pathogens, which often present a great degree of genetic homogeneity.