Pulsed-field gel electrophoresis (PFGE) using different restriction enzymes was used for examining the genetic variability within Chilean Flavobacterium psychrophilum in field strains isolated from farmed Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss). Clustering of the SacI restriction patterns distinguished the Chilean isolates in two genetic groups that were strongly related to the host origin of the isolates (trout or salmon). When the Dice coefficient was applied the similarity between the two clusters was of 87.5%. Restriction digest patterns: resolved by endonucleases BlnI and XhoI did not differentiate the Chilean F. psychrophilum isolates as shown with SacI; trout isolates were clustered together with Atlantic salmon strains. An average band similarity of 83.2 +/- 1.3% was found between all Chilean F. psychrophilum isolates with both. endonuclease. PFGE, regardless of the endonuclease employed, was able to clearly distinguish Chilean F. psychrophilum from the type strain ATCC 49418 T and the Scottish isolate B97026. The results obtained in this work corroborate the existence of a great similarity within this group of Chilean F. psychrophilum strains, indicating that the disease outbreaks in Chilean farms arc dominated by a closely related cluster of strains.