<p><i>Prosopis juliflora</i> was introduced to the Brazilian semi-arid region as a forage and firewood plant and spontaneously dispersed to valley areas and now forms almost pure stands in more than 500,000&#xa0;ha of the region. Since eradication is not being considered, studies on the advantages and disadvantages are necessary. One of the potential advantages is the capability of symbiotically fixing atmospheric N₂. We determined <i>P. juliflora</i> N fixation in six valley areas of Pernambuco state, Brazil. In each area, leaf and wood biomasses were estimated, N concentrations and amounts determined and N fixation in leaves estimated by the ¹⁵N technique. Aboveground and leaf biomasses averaged 44.1&#xa0;Mg&#xa0;ha⁻¹, and 2.1&#xa0;Mg&#xa0;ha⁻¹, respectively. The nitrogen derived from the atmosphere (%Ndfa) in the leaves averaged 38.8%, corresponding to 24.2&#xa0;kg&#xa0;ha⁻¹&#xa0;yr⁻¹ of N fixation. An indirect rough estimation of the average N stock derived from fixation in the woody parts was based on the assumption that the ¹⁵N values in the leaves and wood in the same plant was equal, and amounted to 74.1&#xa0;kg&#xa0;ha⁻¹ Considering the fixed N in the leaves as a stock, adding the estimated N stock in the wood, and extrapolating for the 113 thousand hectares covered by <i>P. juliflora</i> in Pernambuco, Paraíba and Rio Grande do Norte states would be equivalent to a stock of 11.1 Gg of N. Despite being a crude estimate, it indicates that <i>P. juliflora</i> can potentially contribute to a significant amount of fixed N in the plant-soil system in the Brazilian Northeast region.</p>