|Abstract:|| This paper deals with verb stem alternations involving tone, glottalization, and length in the Chin language Zahao (Osburne 1975), spoken in Burma. The emphasis is on tone, but the length facts are extremely interesting, and are given a preliminary treatment in the final section. It is argued that verbs must list stem alternants in the lexicon, but the possible pairs, and the choice of alternant in a given environment, are controlled by markedness. The data offer a theoretical challenge for an output-based grammar like Optimality Theory (Prince and Smolensky 1993), in which the lexicon plays a relatively minor role in the grammar since it is inaccessible to direct control. The Zahao data adds to the list of well-known cases in which allomorphs must be listed in the lexicon, and allomorph selection is then controlled by phonological markedness (e.g. French ma/mon, ce/cet, see Tranel 1998). In Zahao, markedness indirectly constrains the set of possible tonal allomorph paradigms, because positional markedness selects the least marked allomorph in one environment, and pressure to realize the full lexical entry selects the more marked allomorph elsewhere. Output-output constraints limit the permissible difference between the two allomorphs. H and L tones tie on markedness, and exchange rules are analyzed as the result of this markedness parity.