Laboratory, growth chamber and field experiments were conducted to select among 226 isolates of Rhizobium meliloti for the ability to grow, nodulate alfalfa (Medicago sativa L.) and support N2-dependent plant growth between 9° and 12°C. There was wide variation in the abilities of R. meliloti isolates to grow and form nodules at 10°C. Culture doubling times (td) varied from 1 to 155h, and the number of nodules formed on alfalfa in growth pouches in 2 weeks varied from 0 to 3.8 nodules per plant. Nodulation occurred at 9°C, but there was no significant N2-dependent plant growth at this temperature. However, several isolates of R. meliloti had the ability to nodulate alfalfa and produce N2-dependent growth at root temperatures between 10° and 12°C root temperature than did 14 other isolates tested. In field experiments, inoculation with strain NRG-34 resulted in greater nodule numbers, nodule weight, proportion of nodules occupied by the inoculant strain and plant weight than did inoculation with a commercial strain (NRG-185). These results permitted selection of a strain with better low-temperature competitive abilities than the currently available commercial strains.