This paper gives an account of opaque allomorphy in Polish locative in Optimality Theory (OT). Opaque allomorphy seemingly presents a problem to an output-oriented approach, such as OT. This paper provides a solution to opaque allomorphy within OT by the use of contrast. The key idea is that the different allomorphs of the locative suffix preserve contrasts that regular phonology would otherwise neutralize. The analysis is couched within the framework of PC theory (Lubowicz 2003). Predictions of this approach are discussed.