The biology, ecology, and distribution ofSelenicereus (Strophocactus)wittii, one of the least known taxa ofCactaceae, are described. This epiphyte climbs appressed to tree trunks with leaf-like, flattened stems and is found exclusively along the high waterline of black water rivers (Rio Negro, Vaupés, Apaporis) in the Igapó inundation forests of Amazonia. Ecophysiologically,S. wittii is a crassulacean acid metabolism (CAM) plant. It bears white, nocturnal flowers 25 cm in length which emit a fragrance consisting mainly of benzylalcohol, benzyl benzoate, and benzyl salicylate. They exhibit an extreme sphingophilous syndrome as an adaptation to pollination by probably only two species of hawkmoth from the generaAmphimoena andCocytius. The seeds, aberrant for the family, contain air-filled chambers and are water-dispersed. Thus,S. wittii represents the paradoxical life form of an hydrochorous epiphytic cactus which withstands periodical inundation.