This paper examines the well-known alternation between voiced stops and spirants in Spanish. The alternation is standardly analyzed as spirantization of underlying stops; following arguments originally made by Lozano (1979) against this treatment, I propose an OT analysis that treats the alternation as fortition of underlying approximants.Employing Steriade's (1993) Aperture Theory, I propose that a constraint called Strong Onset, demanding that all syllables begin with oral closure, is high-ranked in Spanish, crucially dominated only by Contiguity. This analysis leads to a unified account of this alternation with the similar distribution of the tap and trill in Spanish. The paper concludes with some discussion of issues related to 'underlying form' in Optimality Theory.