Abstract: Rhizobium species F1 was studied for its ability to grow in the presence of trehalose (Trehalose-Minimal Medium (TMM)) and absence of it (Nutrient Broth (NB)) as sole carbon and energy sources and form Trehalose-Catabolic-Enzyme (TCE). The organism was mutagenized with hydroxylamine. The resultant mutants and the parental strain were grown with and without trehalose. The supernatants of the grown culture alone and lysed cells in supernatants were assayed for the activity of TCE. Rhizobium species F1 and the mutants grew in TMM and NB. Many of the mutants grew better (OD670 = 0.36-1.0 in TMM and OD670 = 0.005-0.99 in NB) than the wild-type (OD670 = 0.51 in TMM and OD670 = 0.25 NB). All the strains constitutively and inducibly expressed the trehalose-catabolic enzyme with a range of 0.242-1.42 mg mL-1 glucose mg-1 mL-1 protein for the mutants and 1.025 mg mL-1 glucose mg-1 mL-1 protein for the parental type. In the absence of trehalose in the growth medium, the mutants synthesized higher amount of the TCE with the highest value of 1.091 mg mL-1 glucose mg-1 mL-1 protein and then the wild-type which exhibited enzyme activity of 0.321 mg mL-1 glucose mg-1 mL-1 protein. The enzyme was extracellularly and intracellularly expressed in the TMM and NB. Activity of the total trehalose-degrading enzyme was higher than that of the extracellular. Three classes of the mutants were identified. Low, normal and super-trehalose-catabolic-enzyme producers showed enzyme activity in the ranges of 0 to 30, 31 to 60 and above 60 mg mL-1 glucose mg-1 mL-1 protein, respectively.