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The purpose of this study was to determine the developmental trajectory of the four corner vowels' fundamental frequency (fo) and the first four formant frequencies (F1–F4), and to assess when speaker-sex differences emerge. Five words per vowel, two of which were produced twice, were analyzed for fo and estimates of the first four formants frequencies from 190 (97 female, 93 male) typically developing speakers ages 4–20 years old. Findings revealed developmental trajectories with decreasing values of fo and formant frequencies. Sex differences in fo emerged at age 7. The decrease of fo was larger in males than females with a marked drop during puberty. Sex differences in formant frequencies appeared at the earliest age under study and varied with vowel and formant. Generally, the higher formants (F3-F4) were sensitive to sex differences. Inter- and intra-speaker variability declined with age but had somewhat different patterns, likely reflective of maturing motor control that interacts with the changing anatomy. This study reports a source of developmental normative data on fo and the first four formants in both sexes. The different developmental patterns in the first four formants and vowel-formant interactions in sex differences likely point to anatomic factors, although speech-learning phenomena cannot be discounted.