Nez Perce vowel harmony involves two groups of co-occurring vowels: /i e u/ and /i a o/. Most words contain only one of the two groups of vowels. Certain morphemes are variable and may have vowels of either group, while others have only the second group. The latter type of morpheme, whether stem or affix, dictates the vowel harmony. Three analyses, one in terms of prosody, and two in terms of distinctive features, are given, in an attempt to derive the modern Sahaptian vocalic systems from that of Proto-Sahaptian. 1. The Nez Perce vowels are /i e a o u/. Their norms are [i e a o u m]. /o/ is always rounded; /i e a/ never are. There is considerable individual variation in the degree of rounding of /u/. 2. Among various grammarians of the Nez Perce language, including Smith 1846, Ainslie 1876, Morvillo 1888 and 1891, Phinney n.d. and 1934, Swadesh n.d., and Velten 1943, Morvillo was the first to mention vowel harmony.' Boas, Phinney, Velten, and Rigsby have briefly discussed it,2 and have correctly stated that, with a few exceptions, the set /i e u/ or the set /i a o/ is found in a Nez Perce word.