The essentiality of boron (B) for nitrogen fixation in heterocystous cyanobacteria and rhizobial symbioses has been widely established. However, nothing is known about the possible involvement of the micronutrient in actinorhizal symbioses. Therefore, the effect of boron (B) deficiency on the establishment of the Discaria trinervis‐Frankia BCU110501 symbiosis was investigated. Nodulation was diminished in B‐deficient D. trinervis or in plants inoculated with Frankia grown in the absence of B. These poorly nodulated plants showed a reduction of shoot and root weight and small size. Because depletion of the micronutrient during growth of the actinomycete altered the infection capacity of Frankia, we also studied growth, structure and nitrogen fixation of free‐living Frankia BCU110501. Growth was delayed in B‐deficient BAP media (+N cultures), and completely inhibited in B‐deprived N‐free BAP media (–N cultures), suggesting that B is required to enhance growth of Frankia and essential for the development of nitrogen fixing activity. Ultrastructural study of B‐deficient cells showed an alteration of filament walls both in +N and especially in –N cultures, indicating a possible role of the microelement in the maintenance of these structures. Moreover, the stability of vesicle envelopes was impaired in the absence of B and, hence, nitrogenase occurrence and nitrogen fixation were totally absent. The results show that B is required for both partners to establish an effective symbiosis.