This study was designed to acoustically clarify the speech changes occurring under the condition of DAF (delayed auditory feedback) . The subjects were eight normal male adults without hearing loss or speech disorder. The subjects read a test sentence consisting of about ninety syllables and their speech sounds were recorded. Sonagrams of their speech sounds were made and the mean and standard deviation (articulatooy variability) of the first and second formant frequencies of vowels /a/, /i/ and /o/ were measured. The results obtained in this study were as follows: 1. Neither the mean of the first formant frequencies nor the mean of the second formant frequencies of the vowels showed statistically significant differences between normal and delayed auditory feedback conditions. 2. Standard deviations of the formant frequencies of the vowel /a/ showed statistically significant differences between the two auditory conditions, while no significant differences were found in those of /i/ and /o/. 3. The effects of delayed auditory feedback on vowel articulation appeared to be greater for /a/ than for /i/ and /o/.