The reproductive biology of Pterocarpus macrocarpus Kurz (pradu) was studied in 37-year-old plantation trees in Thailand to determine the causes of seed and fruit loss. Trees flowered at the end of March or early in April at the end of the hot dry season and start of the rainy season. Flowering occurred over about a one-month period. Fruits developed over the next six months during the rainy season and matured at the start of the cool dry season in October and November. Phenology was similar in the four trees that were studied in detail. Racemes averaged 30 flowers each and each raceme was receptive for several days, although each flower was only receptive for one day. After pollination, floral parts were shed over several days and fruits began to develop. Pradu is entomopholous but its insect pollinators were not identified. The stigma is covered by hairs and a secretion is produced. A high proportion of flowers were pollinated. Then, there was a rapid loss of flowers and young fruits. These observations and earlier genetic studies indicate the probability of a high level of self-incompatibility in this predominantly outbreeding species. Pradu may have a very late-acting self-incompatibility mechanism found in many other hardwoods. The zygote remains quiescent for six weeks as the endosperm develops. During this time most of the ovules and fruits abort, suggesting resource allocation preferentially to cross-pollinated ovules. Pradu has a high reproductive potential but a low preemergence reproductive success (0.8), which is common for many hardwood species. The major cause of the low reproductive success was fruit loss during early development. Fruit production may be enhanced by increased cross-pollination among unrelated parent trees. This may be accomplished in seed orchards and seed production areas by the introduction of additional insect pollinators that travel greater distances between trees and by the relatively close spacing of unrelated parent trees.