A study investigated the order of acquisition of inflectional morphology in English within morphemes, focusing on late acquisition of one allomorph of the plural. It is proposed that late acquisition is rooted in the operation of the Obligatory Contour Principle (OCP), which states that at the melodic level, adjacent identical elements are prohibited. The OCP and its operation in English are described. Based on data on acquisition of the allomorph in question, it is argued that the underlying phonological representation of the plural is a single consonant /z/, not a syllable incorporating a schwa, and that the OCP operates to trigger epenthesis of the schwa rather than to block its deletion. (MSE)