A survey of 2- to 8-year-old Acacia mangium plantations in Peninsular Malaysia revealed a high incidence of heart rot ranging from 48.5 to 97.9%. Incidence of rot was correlated with the age of trees but not with site, seed source or pruning treatment. The heart rot occurred either in isolated pockets or throughout the length of the tree bole. Seven morphologically different types of rot recognised were honeycomb rot, spongy rot, spongy rot with zone lines, wet fibrous rot, white fibrous rot, brittle rot and pink pocket rot. Twenty five different Hymenomycete fungi were isolated from these rots. Only Phellinus noxius was always found associated with honeycomb rot. Although the percentage of wood volume loss due to heart rot was generally less than 10, its occurrence especially in the first six metre log possibly affected the quality of sawlogs to be produced from these plantations. In view of the susceptibility of A. mangium to heart rot and the severity of the rot, the Malaysian government is reviewing its Compensatory Forest Plantation Programme to include planting other exotic and indigenous species apart from A. mangium.