The embryology and seed structure of Mayaca fluviatilis was studied. The embryological and structural seed characters clearly support the assumption of a separate familial status of the Mayacaceae. Most of the embryological and seed characters fit well in with those of the other commelinaceous families, but also suggest more reticulate relationships. The Mayacaceae deviates from the other families by the absence of differentiated siliciferous endotestal and sclerotic exotegmic layers. The presence of an exotesta is probably an adaptation to the hydrochorous mode of dispersal.