This paper describes the morphophonological alternation in Mapudungun, caused by the causative <<-(u)m >> to a verbal root ending in /f/, /(sic)/ (and /(sic)/, according to what was consulted in historical sources), segments that are realized as /p/, /k/ (and /(sic)/), respectively. Synchronously, the phenomenon does not affect a natural class of sounds. After a review of historical lexicographic sources of Mapudungun, it was possible to identify four pairs of lemmas that would support that the alternation at some point in the history of Mapudungun was conditioned by phonology. The pairs are: << pur/putrum >>, << lur/lutrum >>, << nor/notrum >>, << trur/trutrum >>. This finding is linked to background information regarding the historical pronunciation of some Mapuche segments that currently present allophones in free variation.