This dissertation is concerned with the linguistic documentation, description and analysis of naturally-occurring discourse in Isthmus Zapotec (ISO 639 code: ZAI), a Central Zapotec language of the Otomanguean stock. As a tonal, verb-initial, and under-documented language, the analysis of ZAI represents a unique opportunity for the cross-linguistic study information structure: the ways that the different components of sentences - constituent order, intonation, morphology, and syntax - are organized by ZAI speakers in order to communicate certain kinds of information. The study is based on a corpus I collected through 17 months of fieldwork. I begin by reviewing the main typological characteristics of the language, including, the tone system, the structural function of prosody, and constituent order, and show that the most common arrangement of constituents in ZAI is verb followed by subject then object. Verb-initial syntax, however, is often violated as the pre-verbal position can be the locus for important discourse functions. I show that the pre-verbal position interacts closely with grammatical role and pragmatic status of nominals in the expression of topic and focus relations. Through the close examination of the form, function, and distribution of ZAI nominals, I analyze the different nominal forms used to introduce and track referents and to mark referents as more or less accessible. I focus specifically on the distribution and alternation of two types of third person pronominal forms, the zero form and the overt subject enclitic form, in spontaneous narrative and conversation, and conclude that an important factor governing their use is the relative thematic salience of the referents: the overt pronoun is used for more thematic figures and the zero form for less thematic figures. I then build on the discussion of nominal forms to address topic and focus relations. I find that while sentence focus and predicate focus constructions are consistently verb-initial, argument focus constructions may contain either pre-verbal constituents (within the clause) or, alternatively, may be verb-initial. No evidence is found for pitch accents directly associated with focal material. I extend the analysis of topic and focus relations by examining data from narrative and conversational contexts. I focus specifically on a multifunctional discourse particle, LA, and argue that LA-marked constructions should be viewed not only as a resource for marking various types of topical information, but more generally as a resource for organizing talk and interaction. I also discuss a conversational strategy in which ZAI speakers use predicate focus and argument focus successively as a chiastic structure in which two intonation units are bound into a couplet to be interpreted together. One effect of this use is to extend a speaking turn for an additional intonation unit, with the second part, the argument focus construction, marking the end of the speaker's turn, ceding the floor. Overall, the analysis offered demonstrates the value and need for information structure studies to document and analyze spontaneous and naturally-occurring discourse, particularly in under-documented languages.