Nitrogen fixation, growth and yield in faba bean (Vicia faba L.) are all adversely affected by water shortage. The objective of this investigation was to examine the effects of water stress on symbiotic nitrogen fixation and plant nitrogen status and to compare yield formation in four different cultivars. Water stress was applied at different times during the growth period to determine any differences in sensitivity due to plant age. Dinitrogen fixation was measured using a non-destructive acetylenereduction assay.Water stress during flowering severely depressed nitrogenase activity and yield formation in all cultivars. Yield was affected to the same extent as with permanent stress from pre-flowering until harvest, indicating that flowering is extremely sensitive to water stress. Water stress during pod filling hardly affected nitrogenase activity and biomass production of the cultivars tested.Although nitrogenase activity was sharply depressed by water stress, plants showed no nitrogen deficiency and in several cases the total nitrogen concentration in stressed plants was higher than in control plants. These results indicate that dinitrogen fixation was not limiting plant growth under water stress conditions. Additional mineral N fertilization neither improved yield formation nor increased total nitrogen concentration, and could not compensate for the negative influence of water stress. It is thus concluded that symbiotic nitrogen fixation is not the limiting factor of yield formation of faba bean under water stress conditions.