Errors in the production of verb inflections, especially tense inflections, are pervasive in agrammatic Broca's aphasia ("*The boy eat"). The neurolinguistic underpinnings of these errors are debated. One group of theories attributes verb inflection errors to disruptions in encoding the verb's morphophonological form, resulting from either a general phonological deficit or a morphological affixation impairment. A second group of theories attribute verb inflection errors to disruptions that arise during sentence formulation, either for syntactic reasons or due to impairments in making fine semantic distinctions between inflectional variants of a verb (+PAST [arrow right] "ate", "hugged"; +FUTURE [arrow right] "will eat", "will hug"). These morphophonological and morphosemantic accounts were evaluated by comparing the efficacy of two treatment protocols that exclusively targeted either morphophonological operations or morphosemantic distinctions. Using a single participant design, it was found that aphasic individuals who received morphosemantic treatment showed significant improvement in accurate production of trained and untrained verb inflections in sentence contexts. In contrast, individuals who received morphophonological treatment failed to show improvements in accuracy of sentence production, although the number and diversity of inflected verbs increased. The differential outcomes suggest that morphosemantic impairments contribute to verb inflection deficits in agrammatic aphasia to a greater extent than morphophonological impairments, at least in this group of participants. (Contains 5 tables and 4 figures.)