Six strains of Escherichia coli, isolated from urine of dogs with urinary tract infection(UTI), were examined to assess of urovirulence factors(UVFs)in the pathogenesis of UTI in an experimental pyelonephritis mouse model.From the results of ID₅₀ and LD₅₀, isolates having different UVFs in the same O serotypes varied in pathogenicity, and isolates having the same UVFs in different O serotypes had nearly the same pathogenicity.Histopathogenic examination revealed that the presence of pap, hly and cnfl contributed greatly to the development of upper UTI.It has also been suggested that hly and cnfl significantly related to the LD₅₀ of the strain in the mouse model, confirming that UVFs are closely related to the pathogenicity of canine UTI.