Larvae of the sesarmid crab Armases angustipes Dana, 1852, captured from the coast of the state of São Paulo, Brazil, were reared in the laboratory from hatching to the megalopa stage. In this paper, the morphology of the four zoeal stages and the megalopa is described and illustrated. Morphological comparisons are made with all known descriptions for the larvae of Armases including an unpublished description of A. angustipes and of some closely related genera (Aratus and Sesarma). Several differences to the previous description of A. angustipes are shown, and additional characters such as the previously undescribed cephalothoracic setation of the larval stages are described and illustrated. Diagnostic characters are provided to differentiate the American genera Sesarma and Armases.