Cupuassu (Theobroma grandiflorum), one of the most profitable crops of Amazonia, is now attracting world-wide attention as an exotic fruit, used in juices, ice cream and sweets. It is a shade tolerant tree that can be grown as a component of agroforestry systems. Nevertheless it is still a wild species and little is known about its biology. Floral biology of cupuassu was studied in Belem-Brazil during 2 floweringfruiting seasons between June 1991 and December 1993. Flowering occurs in the drier period of the year. Flowers commence opening at any time of the day, but open fully at the end of afternoon. The anthers dehisce and the stigmas are receptive as soon as the flowers are fully open. Stigmas remain receptive until 10:00 am the following day. Throughout this period, the pollen grains remain viable. The flowers have a complex morphology which favours allogamy. The species is also self-incompatible. Experimental pollinations, using compatible pollen grains, have shown that a flower which receives 60 compatible pollen grains has 20% probability of setting fruit; a flower which receives more than 400 pollen grains always sets fruit. However, only around 2% of naturally pollinated flowers receive more than 60 pollen grains. A stingless bee, Plebeia minima, is considered an effective pollinator of cupuassu. Another stingless bee, Trigonisca pediculana, also visits cupuassu flowers. Both bees are small insects, which are unlikely to fly very far. Ants (Wasmannia sp.) and weevils (Baris sp.) were considered secondary pollinators, unlikely to promote effective pollinationsFruits mature during the wet period of the year, approximately 5.5 months after the flowers open. The limited and irregular fruit set is probably caused by scarcity of pollinators. The transformation of cupuassu to a plantation crop will therefore require conditions which favour natural pollinators and their access to receptive and compatible flowers