The response to water stress was studied on white clover grown hydroponically. Two varieties (Crau and Huia) were both subjected to a moderate and a more-severe stress, induced by polyethylene glycol (10 and 20% respectively), in the presence of a nutrient solution poor in potassium (K1=0.005 mM), or abundantly supplied (K2=5mM). Dawn water potential and nitrogen fixation (acetylene reduction activity) decreased with the increasing stress. Conversely, the stomatal resistance increased whenosmoticum was added. Crau had a lower stomatal resistance to the deficit, than did Huia. In relation with the K supply, treatment K2 confirmed the superiority of Crau. Crau also showed greater nodule mass and number than Huia. The data show relationships between dawn water potential, stomatal resistance and nitrogen fixation activity.