Relatively little is known about the mechanisms by which the synthesis of periplasmic glucans is regulated. At least two general types of mechanism may beconsidered: 1)the expres- sion of genes coding for biosynthetic enzymes may befavored by lowand reduced by high osmolarity; 2) the activity of rate- limiting biosynthetic enzymes may be regulated by the ionic strength of the cytosol, which in turn responds to the osmo- larity of the medium. Experiments reported here offer strong evidence against thefirst alternative and in favor of the second.