Annual species of Medicago are important pasture legumes in western parts of iran. Their productions are affected by suitable soil Rhizobium meliloti strains and environmental conditions. The principle objective of this study was to find a suitable Rhizobium meliloti strain(s) for Medicago rigidula.

Two experiments: one in the greenhouse and the other one on the field were conducted in 2006 to determine nodulation, and nitrogen fixation of M. rigidula inculated with five Rhizobium meliloti strains (Domestic from Mahabad, M3, M15, M29 and WSM540).

Symbiotic effectiveness was measured by nodule number and nitrogen fixation by subtracting seed N from total plant N in minus-nitrate treatment at the end of the experiment. In the field, nitrogen difference method technique was used for estimation of N₂ fixation. Lolium prennea was selected as a non-N₂-fixing control plant. The Rhizobium meliloti strain "Domestic from Mahabad" was found to be more effective than the M3, M15, M29 and WSM540 in terms of nodulation, dry matter production and N₂ fixation. This may be due to cold resistance of this Rhizobium strain. The estimated N₂ fixed in greenhouse and field was similar. While nodule number is a very important consideration of symbiotic effectiveness it should be combined with total dry matter and total nitrogen in the evaluation of strains. Rhizobium meliloti strai "Domestic of Mahabad" is recommended for inoculation of M. rigidula in western parts of Iran.