In this paper we report an investigation conducted on a brain-damaged patient who presents a selective impairment in processing elements of some semantic categories, in particular animals, fruit, and vegetables. His difficulties appear mainly when naming pictures, but are also present when naming to definition and when retrieving visual attributes of concepts from memory. The pattern of the patient's performance is compatible with an explanation which posits the locus of the impairment at the level of the structural description, considered as the repository of concepts' visual properties. The results are discussed with reference to the constraints they impose on models of normal conceptual representation and processing.