|Abstract:||This paper develops an analysis of Polish vowel-zero alternations, knows as yers, using a variant of Optimality Theory with partially-ordered constraints. The lexically-conditioned phonological alternations are attributed to cophonologies, and lexical indexation is achieved by allowing morphemes to require pairwise relative rankings. The paper shows that the yer alternations follow from a pressure to maintain paradigm uniformity, while variation in the relative ranking of faithfulness constraints accounts for the lexical conditioning. The analysis reveals that morphological structure plays a crucial role in integrating the ranking requirements made by morphemes in derived paradigms. It is proposed that membership in a cophonology is determined by the outermost morpheme in cases of conflict, which explains why a given stem may alternate or not, depending on the suffixal material. The co-phonology also account extends to a class of alternating stems, providing a grammatical explanation for a phonologically-restricted class previously treated as exceptional.