Syllables display symmetrical and asymmetrical properties in two Yoruba dialects. In the asymmetrical dialect, only a vowel with an onset participates in syllable-conditioned processes; an onsetless vowel is syllabically inert. In the symmetrical dialect, a vowel, with or without an onset, participates in syllable processes. It is argued that onsetless vowels are not syllabified in the asymmetrical dialect. Since there is no phonological contrast between syllables with onsets and those without onsets in the symmetrical dialect, all vowels are parsed into syllables exhaustively. Using ideas from Optimality Theory, attested interdialectal variation is shown to follow from different rankings of the same syllable and faithfulness constraints.