The generic name Xenostilbum was introduced by Petrak (1959) for a coremial fungus, collected by H. Sydow in 1937 in Ecuador on a branch of an unidentified tree. The type specimen of X. sydowii Petrak (in Herb. W) was studied. The voluminous description given by Petrak (1959) proved to be correct and complete. The bright synnemata develop in brushes on a basal stroma and are 5-10 mm long and 0.3-0.6 mm broad. The conidiogenous head is slightly darker and broader. The conidia develop singly and apically on filiform, 60-120 μm long and 2-3 μm broad, subapically swollen and attenuated hyphae. Mature conidia are ellipsoidal, 4-celled, composed of two large, golden-yellow central cells and two small, nearly hyaline apical cells, rather thickwalled and measure 36-55 x 13-18 μm. The conidiogenous cells are surrounded by numerous, often branched, 1.5-2 μm broad and up to 250 μm long filaments.