Acacia mangium which appears to be one of the most promising species has been planted over wide areas in the humid tropics, because of its rapid growth rate, robustness and broad range of uses. Although A. mangium had been remarkably free from disease problems, recently it has been reported that several 44 months old thinnings were affected by heart rot. However detailed surveys have not been carried out. To clarify the extent and severity of heart rot in plantations and identify the causal agent, a survey on heart rot was conducted at SAFODA plantations in Sabah, Malaysia. The results of the survey indicated that the incidence of heart rot in A. mangium depended on the tree age. In 4 years old plantations, 10% of the trees were already affected by heart rot and the severity of the incidence increased with the age of trees. Fifty percent of 7 years old trees were affected by the rot. Causal fungi invaded mainly from dead branches and 5 fungal species were isolated from the decayed part of heartwood. It was found that A. mangium trees produced many branches from the age of 3 years and that dead branches remained attached to the stem for 3 to 5 years. Thus there are numerous potential infection courts for heart rot fungi on A. mangium trees. Because of the high incidence and severity of the rot, it appears that A. mangium is very susceptible to heart rot. It was also demonstrated that A. auriculiformis and the hybrid of A. auriculiformis and A. mangium did not exhibit heart rot. This fact suggests that heart rot on A. mangium could be controlled by breeding for disease resistance.