Nitrogen-fixing (effective) nodules from various legume-Rhizobium combinations were analyzed for trehalose and other soluble carbohydrates using gas chromatography and for trehalase activity using biochemical assays. Whereas the bacterial disaccharide trehalose was present only in the minority of the nodules, trehalase activity was found in all of them. Extracts from determinate nodules had a higher trehalase activity than extracts from indeterminate nodules. More detailed studies were done on soybean nodules formed in interactions with two effective and 5 ineffective Bradyrhizobium japonicum strains. Only in effective soybean nodules colonized by the strain 61-A-101 was trehalose a major soluble carbohydrate. Irrespective of the wildtype strains used. effective soybean nodules contained about 10 nkat trehalase g−1 fresh weight, whereas the ineffective nodules colonized by mutant strains derived from these wildtype strains contained 2 to 30 times less trehalase. However, a clear correlation between trehalose content and trehalase activity could not be established.