Abstract: Acid soils are widely distributed in southern China and they significantly limit legume growth and reduce nodulation ability, particularly through the influences of high contents of H⁺ and aluminum in soils. Selecting acid tolerant rhizobium strains is believed to be important for planting Medicago sativa in acid soils. M. lupulina and M. sativa belong to the same genus and their rhizobia can infect each other and form nodules. Field studies have shown that M. lupulina could grow in acid soils, suggesting that its rhizobium might be acid tolerant. Therefore, in the current experiment, rhizobium strains from M. lupulina (R1) and M. sativa (R2) were isolated from the fields in Beibei, Chongqing, and propagated on YMA (Yeast Malt extract Agar) solid culture medium. The responses of growth and activities of antioxidant enzymes of the two strains to aluminum and acidity were studied. There were five aluminum levels (0, 25, 50, 75, and 100 μmol/L), and four pH levels (4, 5, 6, and 7). The OD_A600 of rhizobia isolated from both cultivars reduced significantly after aluminum and acidity stress. The OD_A600 from R1 was significantly higher than that from R2. No obvious growth of rhizobium from R2 was observed at less than pH 4 and 5. With increasing aluminum concentrations SOD activity of rhizobium from R1 decreased initially and then increased, while the activities of CAT, POD and GR significantly decreased. The activities of SOD, CAT and GR of rhizobium from R2 hardly changed under low Al concentrations, but decreased at high Al concentrations. All enzyme activities of rhizobia from R1 and R2 decreased with increasing soil acidity. Enzyme activities of rhizobium from R1 were significantly higher than those from R2. Overall, R1 had better tolerance to Al and acid than R2. Further study is needed to analyse the affinity between Rhizobium strains from M. lupulina and M. sativa.