Two classes of phage K20 resistant Escherichia coli K-12 mutants were obtained. One class of mutants possessed alterations within the ompF gene while the rfa gene cluster, which is responsible for lipopolysaccharide (LPS) synthesis, was affected in the second class of mutants. The OmpF mutants contained substitutions affecting residues localized within the surface-exposed loops 5, 6 or 7. A single deletion mutation resulted in the removal of eight residues of loop 5. These alterations prevented the binding of K20 to cell surface without affecting OmpF's channel activity. One LPS mutant characterized in detail contained an unusual aberration within the rfa gene cluster caused by an IS5 element. Its insertion mediated a deletion encompassing the rfaBIJ genes. Genetic complementation analysis revealed that the rfaB gene, whose product catalyzes the addition of a galactose residue to the first glucose molecule of the LPS core, is necessary to synthesize LPS able to serve as a co-receptor for phage K20.