One striking feature of Vimeu Picard concerns the regular insertion of epenthetic vowels in order to break up consonant clusters and to syllabify word-initial and word-final consonants. This corpus-based study focuses on word-initial epenthesis. It provides quantitative evidence that vowel epenthesis applies categorically in some environments and variably in others. Probabilistic analysis demonstrates that the variable pattern is constrained by a complex interplay of linguistic factors. Following Labov (1972a, 1972b) and Antilla and Cho (1998), I interpret such intricate grammatical conditioning as evidence that this variation is a reflection of a grammatical competence that generates both categorical and variable outputs, and I propose an account within the framework of Optimality Theory. An analysis of individual patterns of epenthesis by members of the community reveals that, even though all speakers share the same basic community grammar, their use of epenthesis differs qualitatively as well as quantitatively. I show that individual grammars can be derived from the community grammar, and that Optimality Theory allows us to formalize the idea that individual grammars constitute more specific versions of community grammars.