The family Cactaceae is widely distributed in South and North America and presents a large spectrum of floral biotypes: melittophily, sphingophily, ornithophily and chiropterophily. The floral biology of Melocactus zehntneri (Br. & Rose) Liietzelb. (subfamily Cactoideae) and Opuntia palmadora Br. & Rose (subfamily Opuntioideae) was studied in a caatinga vegetation, in the district of Alagoinha in Pernambuco, Brazil. M. zehntneri and O. palmadora have tube-shaped flowers, hermaphrodite, without odour, lilac and red respectively. The floral anthesis of O. palmadora is diurnal and the flowers last 48 hours, the nectar is secreted and stored in the nectariferous chamber, and is more copious on the first morning of anthesis (20µl). The sugar concentration of nectar varies between 23 and 30%. Anthesis in M. zehntneri occurs at 14:30h, and the flower remains open even up to 18:30h. The nectar secretion is constant, and is more copious from 15:30h (35µ1), when the visits are more frequent. The sugar concentration of nectar is about 27%. The flowering of the two species overlaps during 3 months, however the peaks of flowering of each species do not coincide. It reduces the competition for the pollinators. The flowers of the two species are typically ornithophilous, and are visited and pollinated by the same species of hummingbird, Chlorostilbon aureoventris. The visits are regular, at intervals of 15 to 30 minutes. The two species are self-compatible.