Non-enterotoxigenic porcine Escherichia coli strains belonging to the serogroup O115 have been associated with septicaemia and diarrhoea. Putative factors important in the pathogenicity of E. coli of serogroup O115 include fimbrial antigen F165, haemagglutination (MRHA), lipopolysaccharide, serum resistance, capsule and production of aerobactin. Using TnphoA transposon insertion mutagenesis, two classes of mutants were obtained from E. coli of serotype O115:F165 with respect to the phenotypic expression of fimbrial antigen F165 and MRHA of sheep erythrocytes: class I, F165-MRHA-, serum resistant; class II, F165+MRHA-, serum resistant. In a chicken lethality model, class I mutants were either virulent or of intermediate virulence, while class II mutants were of intermediate virulence. Alkaline phosphatase activity of class I and class II TnphoA mutants showed similar environmental regulation to that of fimbrial antigen F165. Moreover, class I and class II mutants were mutated in the prs-like locus, and lacked a 18.5 kDa and/or a 17.5 kDa fimbrial band.