The effect of boron deficiency on symbiotic nitrogen fixation in pea (Pisum sativum) was examined. The absence of boron in the culture medium resulted in a decrease of the number of nodules and an alteration of nodule development leading to an inhibition of nitrogenase activity. Examination of boron-deficient nodules showed dramatic changes in cell walls and in both peribacteroid and infection thread membranes, suggesting a role for boron in the stability of these structures. These results indicate that boron is a requirement for normal nodule development and functionality.