Mining, processing and transport of target metals and coal can lead to releases of co-occurring metals such as manganese into coastal waters. Many of these operations are near sensitive coral reefs in South East Asia and Melanesia and Oceania regions. Our previous work has shown that corals are very sensitive to manganese and that the adult life-stage is more sensitive than the juvenile life-stage. However, there are no chronic effects data for adult coral exposed to manganese that can be used for deriving water quality guideline values that provide long-term protection of marine ecosystems. We present the first acute and chronic toxicity data for manganese to adult Acropora millepora. In a collaboration between CSIRO Land and Water, Southern Cross University and the Australian Institute of Marine Science (AIMS), the facilities at the National Sea Simulator, Townsville, Queensland, Australia were used to perform 2-d and 14-d exposures of dissolved manganese to adult A. millepora with coral tissue sloughing used as one of the endpoints to assess coral health. This physiological response involves the disconnection of coral tissue from the skeleton without necessarily causing bleaching. Therefore, toxicity is associated with effects on the animal (host) rather than on the algal symbionts and is irreversible. Other observations were made of bleaching, tentacle retraction and acontia protrusion. An acute to chronic ratio for A. millepora was derived that may be applicable to other coral species. The chronic effects data for adult A. millepora based on measured dissolved manganese were then included in a species sensitivity distribution along with toxicity values for other marine species taken from the literature. Together, these data will ultimately be used to derive guideline values that will provide long-term protection from manganese for 80%, 90%, 95% and 99% of species in marine ecosystems.