This paper examines so-called a-prefixing in the speech of dialect speakers living in Appalachia. Building on existing empirical work, and leveraging results from a GoldVarb analysis, I posit the a-prefix realizes a morpho-pragmatic feature which expresses a range of related meanings, including surprise, unexpectedness, and newsworthiness. This range of meanings is encompassed in the linguistic category mirativity. The prefix is therefore suggested to spell out the feature [MIRATIVE]. Variable insertion of the a-prefix is blocked when the base does not meet certain phonological requirements. In particular, prefixing is blocked with forms beginning with a lax vowel (‘a-ask-ing’), and with forms possessing non-initial stress (‘a-discover-ing’ ). A formal, Optimality-Theoretic (OT) analysis of the interaction of morpheme insertion and phonological markedness is provided.