Speech sound errors exhibited by three conduction and three Broca's aphasic patients on naming and word-repetition tasks were subjected to phonemic and subphonemic analyses. In the conduction aphasic patients, errors occurred equally often on consonants and vowels in both the naming and word-repetition tasks, while in the Broca's aphasic patients errors occurred selectively on consonants. Transposition errors occurred almost as often as substitution errors in the conduction aphasic patients, while substitution errors constituted the majority of errors in the Broca's aphasic patients. The Broca's aphasic patients, as compared to the conduction aphasic patients, exhibited a markedly higher number of substitution errors occurring between phonemes separated by a single subphonemic feature on the naming task. On the basis of these findings, it was hypothesized that the differences in the error patterns of the two types of aphasia reflected differences in the underlying mechanisms of the impairment in each type.