The dorsal valve of a Pleistocene terebratulid brachiopod, Terebratula scillae Seguenza, 1871, has developed a malignant cyst due to colonisation in vivo by an endolithic sponge. This trace fossil is a compound boring and bioclaustration structure, representing a boring that has grown in unison with the growth of the cyst. The brachiopod has grown to adult size and growthlines indicate that it was colonised by the sponge when about half grown. Malformation of the shell may not have caused the death of the brachiopod and the sponge does not appear to have outlived its host; both symbionts seem to have died more or less simultaneously. This minus-minus relationship of two symbionts is considered to be a case of ‘accidental symbiosis’