Rhotacism in Latin is a well-known phonological generalisation which, in its paradigm cases, can be stated as a regular sound change of [s] to [r] between vowels. This change/rule is posited on the basis both of comparative evidence, e.g. *swesor Latin soror ‘sister’, and of paradigms in which final s alternates with medial intervocalic r, e.g. flos, floris ‘flower’. It is possible, however, to cite a number of exceptions to the basic rule which, if one attempts to account for all of them in a synchronic grammar, amount to outright paradoxes. This paper presents a diachronic model of the progress of rhotacism through the expected life cycle of a phonological process, within the formalism of Stratal Optimality Theory, and demonstrates that this model can account for the exceptions to rhotacism as epiphenomena of the expected progress of the constraint ranking giving rise to it from phrase to word level, and from word to stem level. Finally, I argue that rhotacism became a systematic property of the lexicon, at which point it was subject to analogical extension, giving the paradigm levelling observed in e.g. honor, honoris ‘honour’ (formerly honos, honoris).