Fimbrial adhesins and hemolysins contribute to pathogenicity of extraintestinal Escherichia coli isolates causing urinary tract infections (UTI), sepsis and new born meningitis (NBM). Using gene cloning techniques and pulse field electrophoresis in combination with Southern hybridizations it was demonstrated that the genetic determinants coding for P and 'P-related' fimbrial adhesins and hemolysins are closely linked on the chromosomes of different pathogenic E. coli wild-type isolates. For two UTI strains, 536 (O6:K15) and J96 (O4:K6), a co-deletion of the linked gene clusters coding for hemolysin and fimbriae was observed. The deleted DNA regions which also comprise flanking DNA sequences were termed 'pathogenicity DNA islands'. Such 'pathogenicity DNA islands' were also detected in the genome of O18:K1 isolates of OMP type 6 but were absent on the chromosomes of O18:K1 strains of OMP type 9. A mutant strain, 536-22 was selected from rat kidneys after intraurethral infection of animals with the wild-type parental strain 536. This particular isolate also shows deletions of 'pathogenicity islands' leading to a non-pathogenic phenotype. It is therefore concluded that excisions of 'pathogenicity islands' from chromosomes of pathogenic E. coli strains are not restricted to the laboratory but also occur in vivo. The generation of deletions may represent a general mechanism of bacterial virulence modulation.