Because arbuscular mycorrhizal fungi (AMF) has been shown to induce concentration increases of pharmaceutically useful phytochemicals in some Brazilian semi-arid native plants, the current study examined whether mycorrhizal inoculation increased the production of bioactive compounds, especially gallic acid, in field grown Libidibia ferrea (Mart. ex Tul.) L. P. Queiroz plants. Seedlings were inoculated with Claroideoglomus etunicatum (W.N. Becker & Gerd.) C. Walker & A. Schüßler, Acaulospora longula Spain & N. C. Schenck, Gigaspora albida N. C. Schenck & G. S. Sm. or non-inoculated (control) and, seven months after transplanting, examined for growth parameters, chlorophyll, phenols, tannins and gallic acid concentrations, mycorrhizal colonization and rhizosphere AMF spore density. Plants inoculated with C. etunicatum had 21% higher gallic acid concentrations than control plants while those inoculated with G. albida had higher total chlorophyll concentrations. Mycorrhizal technology employing C. etunicatum can constitute an alternative to increase gallic acid production in field grown L. ferrea plants.