This paper examines a previously identified but so far imprecisely defined split in the historical phonology of Resígaro (Arawakan). I argue that this development was a simple case of allophonic redistribution or primary split, in which word-final allophones of *a merged with ɯ. However, comparative data offers no evidence for the existence of ɯ before the operation of the split in question. This apparent paradox can be resolved by the plausible hypothesis that ɯ entered the language via the massive influx of morphemes from Bora, an unrelated language of the Bora-Muinane family. Only after that were some contextual allophones of *a merged with ɯ. This is the first solidly justified case of loan phonology in Resígaro and it provides further support to the hypothesis that Bora influence on Resígaro is not a result of the language's obsolescent status.