Fast-growing acacias are being used to reforest degraded grasslands and cutover rainforest in the humid tropics. An ACIAR-sponsored project was established to investigate the floral biology of Acacia mangium and A. auriculiformis as the basis for the manipulatied production of hybrids between selected trees, and to evaluate the potential for hybrid seed production in orchards. These proceedings arise from a workshop held in Tawau, Sabah, Malaysia in July 1991.