Alfalfa (Medicago sativa L.) cultivars with increased nitrogen fixation potential would be beneficial. The desirability of existing cultivars as genetics source material for a breeding program to accomplish this objective has not been reported.Ten alfalfa cultivars contrasting in fall dormancy, date of release, and germplasm origin were evaluated as 10‐week‐old seedlings for indices of N2‐fixation under nil‐nitrate sand culture in a lathehouse during the summer months. Largest variability for indices at N2‐fixation was among plants within cultivars and not among cultivars. The results indicated that all of the cultivars evaluated would be equally desirable as source material to improve N2‐fixation potential in alfalfa. No effects were noted which could be attributed to fall dormancy, age of the cultivar, or germplasm source.