Yaminahua (Pano: Peru) has a number of morphophonological alternations that are reportedly sensitive to syllable count. These alternations might target vowels, consonants and syllables. This paper shows that these alternations are metrical and that they arise through the interaction of various metrical, syllabic and alignment factors. Specifically, a tendency for stem disyllabicity and alignment of certain morphemes to the right or left edge of a foot conflicts with a drive to avoid unparsed and onsetless final syllables. The resolution of these conflicting tendencies might cause epenthesis, deletion, reduplication and vocalic alternations in stems and affixes. An analysis in Optimality Theory is proposed in which alternating morphemes are sets of allomorphs in the input and competing metrical, syllabic and alignment constraints decide among the allomorphic variants. An important part of the analysis is a gradient alignment constraint which is crucially unranked with respect to Parse. The Yaminahua data and its analysis contribute to the understanding of the phonology and morphology interface.