Columnaris disease caused by Flavobacterium columnare is a problem in fish farming worldwide. During the last 15 yr, outbreaks have started to emerge in Finland. Flavobacterium columnare Type Strain NCIMB 2248T and 30 Finnish F. columnare isolates were studied using analysis of 16S rDNA by restriction-fragment length polymorphism (16S RFLP), length heterogeneity analysis of polymerase chain reaction (LH-PCR) products, automated ribosomal intergenic spacer analysis (ARISA), and 16S rDNA sequence analysis. All isolates fell into RFLP Genomovar I and had the same length in the LH-PCR analysis. Based on ARISA, 8 genetically different strains were selected for further analyses. The growth of these strains under different temperatures, NaCl concentrations, and pH values was tested. The Finnish F. columnare strains did not grow at NaCl concentrations >0.1% or at pH values < or = 6.5, and they were susceptible to several antimicrobial agents, but not to Polymyxin B or neomycin. These findings may aid in development of methods for disease management at fish farms.