The Island of Combu-PA, 1.5 km from Belém, is characterized as a floodplain ecosystem, where a floristic survey was carried out and 17 species of Arecaceae were registered, among them Bactris major Jacq., commonly known as "marajá-açú". Because most of the studies on this family, especially on the species of the Amazon region, are directed to phytosociological and ethnobotanical surveys and few on the plant anatomy, this work aimed to know the external morphology and the anatomy of the pinnae of this species. For the morphological studies dendrometric measurements were made in the field and for the anatomical studies, median pinnae were collected and sectioned and fixed in 70% alcohol to be submitted to the usual techniques of plant anatomy. The external morphology showed that B. major Jacq. occurs in clumps, with stipes 2-8 m tall and 2-6 in diameter, has rigid and flattened spines on the internodes; intrafoliary inflorescence of the prophyllous type with unisexual flowers; infructescence in bunches with obovoid dark brown fruits when ripe. The anatomy of the pinnae revealed that in this species, in frontal view, the epidermal cells are tangential and siliceous bodies occur in each vertex; hypoestomatic lamina with hexacitic stomata. In transverse section the pinnae showed a hypodermis near the abaxial and adaxial faces; dorsiventral mesophyll showing nests of non-lignified fibers surrounded by siliceous bodies with triangular shape, presence of columnar brachiesclereids and vascular elements involved by lignified fibers. By the morphological description, habit and comparisons with existing works there is a tendency to consider the species studied as a variety, Bactris major varo major Jacq. As for the anatomy of the pines it was possible to verify that this variety presents very accentuated mechanisms of support due to the presence of lignified fibers, siliceous bodies and sclereids
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