Welsh vowel mutation is a purely positional vowel alternation, the effects of which serve to obscure phonemic contrasts between three vowels in the system, namely barred-i, schwa and [u]. The theoretical interest in this alternation stems from the surface orientation of current phonological theory: is such a non-surface-true state of affairs amenable to plausible modelling in an optimality-theoretic framework, or are the relevant relationships best accounted for through lexical listing? In this paper I argue that a straightforward optimality-theoretic account is available and that this account is simpler than any of its predecessors. The analysis differs from previous derivational analyses (e.g. Thomas 1979, 1984; Williams 1983; Bosch 1996) in various ways, including the underlying values of some of the vowels involved, the avoidance of ad hoc extrinsic rule ordering, and the lack of reliance on intermediate representations. Furthermore, reference to phonological position alone is sufficient, with no need to refer either to stress or to morphological complexity. The correct results emerge primarily through the interaction of a high-ranking structural constraint prohibiting schwa in a final syllable, an input–output faithfulness constraint on vowel features, and a constraint prohibiting a high central rounded vowel, [u with bar through].