Summary. Alternaria argyroxiphii is reported for the first time as the cause of leaf spot on African mahogany trees (Khaya senegalensis). The disease was first observed in African mahogany field plantations in Perdoes, Minas Gerais, Brazil, in October, 2012. Morphological characteristics and phylogeny of the pathogen were determined, and pathogenicity assays assessed the species specificity. Leaf spot was the major symptom observed. Phylogenetic analyses of the combined dataset of the Alternaria major allergen gene (Alt a 1) and glyceraldehyde-3-phosphate dehydrogenase (gpd) showed that the recovered isolates clustered into Alternaria sect. Porri and much resembled A. argyroxiphii, with 99% booststrap support and Bayesian posterior probability of 1.0. This was largely concordant with morphological characteristics (on potato carrot agar) of A. argyroxiphii, including large conidia and long beaks. A pathogenicity test, carried out to fulfil Koch's postulates, confirmed A. argyroxiphii as a causal agent of leaf spot of K. senegalensis. Key words: leaf disease, phylogenetics, pathogenicity.