ABSTRACT Acrostichum danaeifolium, a Neotropical fern, occurs preferentially in marshy areas or at the margins of lakes and mangroves. Microlepidoptera larvae burrow through the petioles of the fern, preferentially on the non-expanded leaves. The galleries in the petiole create a new microhabitat, harboring a rich fauna of arthropods. The aim of the present study was to assess the richness of ants associated with its petiole. The study was conducted in a population of A. danaefolium from the Atlantic Forest in Rio de Janeiro state, Southeastern Brazil. Six collections were carried out every two months (2009-2010), three in the dry and three in the rainy season. The leaves were divided into three development stages: non-expanded leaves (NEL), expanded leaves (EL) and senescent leaves (SL). Seven leaves from each phase were randomly collected from seven individuals. A total of fifteen ant species were recorded. The species with the highest frequency and density in fern petioles were Camponotus crassus and Crematogaster curvispinosa. The highest ant richness and abundance was found in senescent leaves. The high number of ants found in the petioles of Acrostichum danaefolium qualifies it as a potential key species in the marshes and flooded areas where it occurs.