When Erwinia chrysanthemi was grown in a sodium polypectate/yeast extract/salts medium adjusted with sorbitol to water activity (a,) values of 0.990 and 0.980, extracellular pectate lyase (PL) production was repressed, whereas intracellular PL levels were not affected. Inorganic solutes (NaCl, LiCl, KCl, Na2S04 and a NaCl/KC1/Na2S04 mixture) at 0.990 a, strongly stimulated the intracellular levels of PL (6-to 17-fold greater than the control), whereas extracellular levels were only slightly increased. Intra-and extracellular PL levels were not affected by LiCl, NaCl and KCl at 0.995 a,. A NaCl/sorbitol mixture (0-990 a,) completely inhibited extracellular but not intracellular PL production. Lowering of the a, of cultures during mid-exponential growth phase with NaCl(O.990 a,) resulted within 30 min in a 70-fold increase in intracellular PL activity. When sorbitol was used in similar experiments, extracellular PL production was inhibited to a greater extent than intracellular levels.