The article deals with irregularities in the morphological make-up of Italian verbal forms, focusing on perfect and past participle forms. It aims to account for root-based contextual allomorphy in the framework of Distributed Morphology (Halle & Marantz 1993). Building upon the generalisation that morphological irregularities result whenever the thematic vowel is absent, the article provides a synchronic account and a diachronic analysis by means of a restricted set of morphophonological rules, thus challenging both the traditional view, according to which morphological irregularities follow from stress positioning, and paradigm-based accounts