Acrostichum aureum and A. danaeifolium are morphologically similar sympatric species which grow in mangrove communities. To evaluate the cytological differences between these species, their karyotypes were analyzed with conventional staining, triple-staining with chromomycin A3 (CMA), distamycin A (DA) and DAPI, silver nitrate, and in situ hybridization with 45S rDNA as probe. Both species have the same chromosome number (2n = 60) with only small differences in chromosome size and morphology. The CMA banding pattern revealed four terminal bands in A. danaeifolium and six in A. aureum. DAPI bands were not found. The maximum number of nucleoli per interphase nucleus and the number of 45S rDNA sites were consistent with the number of CMA bands: four in A. danaeifolium and six in A. aureum. All meiotically analyzed materials showed 30II with normal chromosome pairing and segregation, except in one plant with a chromosome bridge and fragment in cells of anaphase I and II. It is suggested that sympatry and karyotypic orthoselection have contributed to keep the morphological and karyological similarities in such widespread species.