Two aspects of the duration control of vowel production were examined in deaf and normally hearing subjects: inherent durational differences between two closely related vowels (/i/ and / i /) and the modifying influence of the following consonant. Twelve deaf and five normal adolescents pronounced 56 CV(C) words containing a vowel, /i/ or / i /, followed by /t, s, n, d, z, ø/. Whereas in normal speech the tense vowel /i/ is only relatively longer than the lax vowel / i /, in the speech of the deaf subjects these two vowels occupy much more restricted durational ranges. The modifying influence of a following sound is different in the case of the deaf subjects. Differences between the deaf and normal subjects do not appear to be simply errors of speech production, but reflect instead a different type of linguistic structure. The vowel production characteristics of the deaf subjects account in part for the low intelligibility of consonantal sounds in the speech of the deaf, and imply that standard articulation tests may be an ineffective means of assessing speech production processes of deaf speakers.