The development of differential vowel duration was observed in six children who were tape recorded at 1-month intervals from 26 to 36 months of age and in three children from 21 to 24 months of age. By differential vowel duration is meant the relatively different durations of vowels according to whether the following consonant is voiced or voiceless, stop or fricative. The children's task was to produce a series of consonant-vowel-consonant English words using each of the vowels /I i u/ before one voiceless and one voiced fricative. Four stimulus conditions were used: a visual stimulus using familiar storybook pictures and three sets of tape-recorded stimuli with the vowel /i/. One set had normal differential vowel duration, another had abnormal equal vowel duration, and the third had four occurrences of each word with graded vowel durations. Results showed that acquisition of differential vowel duration preceded control of the voicing feature which conditions it in adult English. Correct intrinsic vowel duration was produced in all responses. This document, Part 1, includes the introduction and reports of three stimulus conditions. The report concludes with Part 2, which can be found in document RE 003 492. Tables are included. (Author/DH)