Sponges play an important role in biogenic coral-reef degradation, and it is acknowledged that elevated levels of sponge erosion commonly indicate poor health of coral-reef environments. An increase in the abundance of coral-excavating sponge has been reported from several locations, a development that may move coral-reef carbonate budgets increasingly towards net erosion. The role of coral-excavating sponges on Indian reefs has not been studied in as much detail as elsewhere. The present paper describes the observation of a coral-excavating sponge from the family Clionaidae. This brown, endolithic sponge formed a coherent thin layer over the surface of the substratum and had a spicule complement of tylostyles and spirasters. Therefore it belongs to the Cliona viridis species complex, which, as a group, is widely distributed and commonly displays high bioerosion rates. Accurate identification will require molecular studies and is presently deferred. The sponge was found excavating only Turbinaria mesenterina colonies of Gulf of Mannar. Within the surveyed area of 60 m 2, 38.58% of T. mesenterina colonies were found to be invaded by the sponge. Targeted long-term studies across a larger spatial scale are warranted to assess the role of this sponge in more detail, and whether its abundance changes over time.