Abstract With the purpose of increasing the currently available data on reproductive biology about Neotropical Cuscuta species in general, and mode of reproduction and seed ontogeny in Cuscuta obtusiflora H.B. and K. in particular, both developing flowers and seeds have been analysed by means of standard photomicroscopy together with histological techniques, stereomicroscopy, fluorescence microscopy and field observation. Meiosis of a single archesporocyte originates linear megasporic tetrads with chalazal viability. The megagametophyte belongs to the Polygonum type. Porogamous fertilisation occurs prior to perianth opening. From the apical proembryonal cell ca the rudimentary coiled embryo proper originates, formed mostly by the hypocotyl. A broad suspensor is derived from the frequently vesicular basal cell cb. The haustorial nuclear endosperm becomes divided into cells in a centripetal way. The central cellular endosperm becomes reabsorbed. The probable post-hydration expansion of both a gelatinised remaining endosperm and a scanty tegumentary parenchyma could help fracturing the exotestic seed-cover during germination. At the three-layered testa hydrochoric adaptations have been observed and deemed as related to wetland habitats of such species. Our results suggest that in predominantly cleistogamous populations of C. obtusiflora, gene flow is likely performed through hydrochoric seed dispersal. Preanthesis cleistogamy is likely to increase reproductive performance of this species. Such a pollination system is firstly evidenced in parasitic plants in the present report.