[en] N₂ fixation in vetch (Vicia Sativa) grown alone and in mixture with barley (Hordeum vulgare) in pots was evaluated using ¹⁵ N isotope dilution method. Two harvests were made over the growing season. The proportion of the above-ground N derived from atmospheric N₂ (% Ndfa) in mixed vetch was significantly higher than that of vetch in pure culture for the different harvests. However, this increase was not accompanied with the amount of N₂ fixation. On the other hand, no significant differences were observed in N-content or in N-derived from soil between barley grown in mixture and in mono culture, in spite of the difference in the number of plants between the two cropping systems. These results indicate a high competing capacity of barley for soil nitrogen in mixed culture. So, in further investigations the vetch: barley seeding ratio must be considered in order to reduce the competition effect. Barley grown together with vetch had lower atom %¹⁵ N excess than barley grown in mono culture because it may have received some of their N from N-released by the legume in the second herbage but not in the first. On the other hand, nitrogen accumulation in the two successive harvests was higher than that in the plants harvested only once. Overall, these results indicate the importance of mixed crops for forage production. Based on these results, further investigation must be carried out in the field especially under rain-fed conditions. (author). 6 refs., 2 figs., 3 tabs