The corallivorous crown-of-thorns starfish (COTS), Acanthaster planci , is recognised as a major cause of coral reef degradation throughout much of the Pacific Ocean. However, the effects of COTS on the high diversity reefs in Indonesia have been largely overlooked. In 2007, high densities of COTS were observed in two regions of Indonesia: Aceh and Halmahera. Densities of COTS ranged from 0 to 52 starfish 2,000 m 2 across 24 sites in Aceh and from 0 to 18 starfish 2,000 m 2 at 10 sites in Halmahera. Mortality rates of Acropora spp. were very high at affected sites: over 50 % of colonies had been killed at seven of the 16 affected sites. A review of historical sources going back to 1969 suggests that COTS have damaged many reefs throughout Indonesia, including much activity within the Indonesian section of the Coral Triangle. Furthermore, the data suggest that COTS activity has increased rapidly since 2000. Very little of this activity has been reported in the primary literature, and there is a general lack of awareness in Indonesia of COTS as a potential cause of reef degradation. This lack of awareness, combined with limited monitoring efforts, means that damage caused by COTS is often attributed to other causes, such as destructive fishing, bleaching or tsunami. COTS are clearly a major source of coral mortality in Indonesia of which scientists and government need to be more cognizant.