The HU protein of Escherichia coli has been implicated in various site-specific recombination reactions. Moreover, recent data suggest that HU may also participate in homologous recombination. In particular, it has been shown that P1 transduction is inhibited in the absence of HU [Kano and Imamoto, Gene 89 (1990) 133-137]. In contrast, we found that transductional recombination and conjugational recombination were almost normal in hupA hupB mutants. However, it appeared that the recombination proficiency of hupA hupB mutant bacteria was reduced tenfold in an intrachromosomal recombination assay. Moreover, we found that intrachromosomal recombination was reduced tenfold in a gyrB226 strain and by more than 100-fold in an osmZ205 strain. The gyrB226 mutation affects the DNA gyrase activity, while mutations in osmZ are highly pleiotropic, affecting the expression of a variety of genes and increasing the frequency of site-specific inversion events. Since it has been shown that the hupA hupB mutations, like the gyrB226 mutation, decrease the level of DNA supercoiling, whereas the osmZ205 mutation increases the level of DNA supercoiling, it appears that the histone-like proteins HU and OsmZ may play a key role in intrachromosomal recombination by affecting the DNA topology.