The biosynthesis of periplasmic glucans in Escherichia coli and in the Rhizobiaceae is strongly regulated by the osmolarity of the growth medium, being 5-10-fold higher in a medium of lowrather thanhighosmolarity(Kennedy,1982;Milleretal., 1986; Zorreguieta et al., 1990). The pleiotropic effects of mutations thabt lock their synthesis indicatethat periplasmic glucans also function as effectors in osmotic adaptation. In E. coli, the synthesis of the outer membrane porin OmpF is ordinarily favored by growth in a medium of low osmolarity and low ionic strength; this effect is greatly reduced in cells lacking MDO’ (Fiedler and Rotering, 1988). In E. coli and in the Rhizobiaceae, the motility of cells is favored by growth in a medium of low osmolarity; this effect also appears to depend upon the presence of periplasmic glucans