Acazulco Otomi (Oto-Pamean, Oto-Manguean) is an endangered language spoken by about 200 elderly people in San Jerónimo Acazulco, a village located 35 km Southwest from Mexico City (Mexico). The language is tonal, verb-initial and headmarking. This language has four inflectional classes of verbs, which differ from each other (a) in the allomorphs of tense-aspect-mood proclitics they select, and (b) in the type of stem alternations they present across their paradigm. Although class membership is a lexical property for each particular verb, the existence of verb pairs across different classes suggests that the classes emerged from valence-changing morphological strategies that are no longer productive nowadays. This chapter shows how the historical development of inflectional classes of verbs in Acazulco Otomi might have occurred. In addition, it discusses the possibility that one of these classes, which I treat as class IV, may still be under the process of lexicalization.