The effects of the attenuation of second and third formant frequencies on the recognition of stop consonant vowel syllables (/p,b,t,d,k,g/ paired with the vowel /a/) in fluent and nonfluent aphasic and nonaphasic normal subjects were examined. Statistical analyses revealed significant differences between groups, between formant conditions, and between stop consonants. All groups responded similarly under the formant conditions, but the attenuation of F2 appeared to account for the most striking disparity between consonant recognition scores irrespective of group. A series of confusion matrices showed that confusions made by aphasic subjects involved both place of articulation and voicing errors, particularly for nonfluent aphasics, and involved the substitution of more anterior consonants. Important cues for the voicing of anterior consonants appeared to be carried by F2. The finding that more intact speech production abilities appeared to facilitate more accurate speech recognition was interpreted as support for the motor theory of speech perception.