The internal structure of numeral systems can shed light on processes of word formation, language contact and change. In this article I analyse the numeral system of Purepecha, a language isolate spoken in Michoacán (western Mexico), on the basis of historical and contemporary sources. The Purepecha system is unusual both typologically and areally since (i) it possesses monolexemic terms up to six, while seven to nine are compounds with five; and (ii) the forms for the base (20) and next power (400) have clear non-corporeal meanings, related instead to a configuration of objects and the notion of 'living', respectively. These features render the Purepecha system atypical in the Mesoamerican context, even if the vigesimal base is usedperhaps inaccurately-as grounds for its inclusion in the linguistic area. On the basis of Colville (1985) I provide a more nuanced classification of vigesimal counting systems in Mesoamerica, highlighting the importance of regional interaction in West Mexico as a possible alternative to previously proposed unidirectional diffusion scenarios. The language-internal and external findings underline the importance and utility of more detailed historical studies of Purepecha.