[en] We report a cognitive investigation of a case of deep dysphasia appearing in the context of primary progressive aphasia. Over a period of 5 years, patient CO presented progressive difficulties in word finding and in oral comprehension, while nonverbal cognitive functions remained preserved. As in other deep dysphasic patients, CO's repetition performance showed marked imageability and lexicality effects, and semantic paraphasias. The same effects were observed in writing-to-dictation. Regularisation errors occurred in word reading. CO's short-term memory span was less than two words. A cognitive analysis of language processing revealed difficulties in phoneme identification and rhyme judgement, in detecting grammatical class for orally presented words, and in oral and written naming. The interpretation of CO's deep dysphasic symptoms within interactive models of language processing confirmed the importance of a phonological short-term storage impairment as an explanatory factor of deep dysphasia.