Abstract: Spanish present subjunctive verbs differ from their indicative counterparts primarily in a change in theme vowel. Indicative theme vowel /a/ becomes /e/ in the subjunctive, /e/ becomes /a/, and /i/ becomes /a/ (e.g., ind. /amamos/, 'we love' becomes subj. /amemos/, whereas ind. /tememos/, 'we fear' becomes subj. /temamos/). I suggest that this pattern results from a constraint against homophony between indicative and subjunctive forms, combined with paradigm uniformity constraints and a few particular properties of the Spanish verbal system. My analysis is cast in McCarthy's (2002) Optimal Paradigms framework, a version of Optimality Theory in which candidates consist of entire inflectional paradigms built around a given stem.