Restriction fragment length polymorphism of ribosomal RNA genes was analysed among 133 Escherichia coli strains predominantly from blood and urine, including 21 isolates from faeces of healthy persons. The strains had also been characterized for their O:K:H serotypes, for the presence of P, S and type 1C fimbriae, non-P, non-S mannose-resistant haemagglutinins and haemolysin production. Hind III-digested genomic DNA was subjected to Southern blot analysis with either plasmid pKK3535 containing E. coli rRNA operon or purified rRNA as a probe. Among the 133 strains 20 ribotypes were obtained. The distribution of strains into different ribotypes generally correlated with their O:K:H serotype. Ribotype variation within serotypes was mainly seen among strains with the K5 capsule. The origin of the strains or the presence of virulence-associated factors did not correlate with the ribotype. In conclusion, ribotyping appears to be a valuable method in epidemiologic studies especially when the serotyping methods are not available.