Escherichia coli cells lacking the histone-like protein HU form filaments and have an abnormal number of anucleate cells. Furthermore, their phenotype resembles that of rfa mutants, the well-characterized deep-rough phenotype, as they show an enhanced permeability that renders them hypersensitive to chloramphenicol, novobiocin, and detergents. We show that, unlike rfa mutants, hupAB mutants do not have a truncated lipopolysaccharide but do have an abnormal abundance of OmpF porin in their outer membrane. While the complete absence of HU does not abolish the osmoregulation of OmpF protein synthesis, the steady-state level of micF RNA, the negative regulator of OmpF, decreases in bacteria lacking HU, increasing the basal level of this membrane protein. These findings demonstrate a novel link between a bacterial chromosomal protein and the outer membrane composition.