Seven rhizobium fredii strains and seven Bradyrhizobium japonicum strains were grown in defined medium with or without 20mm trehalose in the presence or absence of NaCl. Trehalose had no effect on the growth rate of the strains in the absence of NaCl, but increased the growth rate of some strains in the presence of NaCl. Bradyrhizobium japonicum strain RCR 3827 was completely inhibited by 0·08m NaCl in absence of trehalose, but multiplied when trehalose was added. The results indicate that trehalose may act as an osmoregulator in these strains of Rhizobium and Bradyrhizobium. Copyright © 1990, Wiley Blackwell. All rights reserved