Modifications in the ultrastructure of the exoskeleton of the atrypid brachiopod Desquamatia subzonata Biernat indicate that, for the greater part of its life, it accommodated the trace fossil Diorygma atrypophilia Biernat. When these modifications are compared with the ultrastructure of the exoskeleton of Hemothiris psittacea (Gmelin), which has been attacked by a boring clionid sponge, it is evident that the ‘burrow’ occupied by Diorygma was not excavated by physical or chemical digestive processes, but fashioned by simultaneous growth of brachiopod shell and occupant. Because of its morphology and inferred habits Diorygma is best placed within the Phoronida.