This paper provides a Harmonic Serialism analysis of the diachronic process of rhotic metathesis that has applied in Algherese Catalan. It gives some empirical evidence for considering the left edge of the stem a prominent position. It is also argued that the typological predictions that derive from Harmonic Serialism are more restrictive than those derived from parallel OT. Using only Harmonic Serialism, long-distance metathesis can be discarded as a potential process of natural languages. It is also proven how, again using only Harmonic Serialism, the two rhotic metathesis patterns found in Algherese Catalan can be accounted for in unison.