The uropathogenic Escherichia coli wild-type strain 536 produces S-fimbriae, P-related fimbriae and type I fimbriae. Using immuno-colony dot and ELISA techniques, variants were detected showing an increased degree of S-fimbrial production. It was demonstrated by immunofluorescence microscopy that in normal (wild-type) and hyper-S-fimbriated E. coli populations non-fimbriated cells also exist, and that the percentage of S-fimbriated and non-fimbriated bacteria was roughly identical in either population. Hyper-S-fimbriated variants could be stably maintained. The transition from wild-type to hyper-S-fimbriation, which occurs spontaneously, is markedly higher than vice versa. Southern blot analysis of the S fimbrial adhesin (sfa) determinants of normal and hyper-fimbriated strains revealed no marked difference in the gene structure.