Average durations of 12 vowels of American English measured in bisyllabic nonsense utterances are reported. The vowels occurred in 14 symmetrical consonantal environments and the utterances were produced by three male talkers. The consonant environments consisted of the voiced and voiceless versions of three stop, one affricate, and three fricative consonant articulations. Four determinants of the characteristic durations of stressed vowels are identified and discussed. The hypothesis is advanced that the primary lengthening of vowels in English—that found in tense vowels and in vowels before voiced constants—is a part of the phonology of the language and is learned by speakers of the language, and that the secondary lengthening of vowels in English—that found in open vowels and in vowels before fricative constants—is a function of the articulatory process itself.