Koasati is an American Indian language of the Muskogean family, presently spoken by about three hundred people in rural southwestern Louisiana. The morphological processes of the language are primarily agglutinative, with limited morphophonemic alterations. The verbal system is typified by three major conjugations, defined by the variation in form of the elements cross-referencing the verbal subject. These subject cross-reference markers have variant forms which reference negation as well as person. There is an elaborate set of verbal prefixes and suffixes, which reference numerous notions, such as location, direction, direct object, dative/benefactive object, adverbial concepts, aspect, tense, and source of information. Like other Muskogean languages Koasati has a system of reference indexing, or switch-reference, which consists of verbal suffixes. In addition there is a series of connective words which also have switch-reference functions. The nominal system contains a moderately well-developed case system, with the following cases: nominative, accusative, autonomous, locative, allative, inessive, and vocative. In addition there are article-suffixes which indicate that a noun has been previously mentioned, and which mark tense on the noun, as well as nominal suffixes which serve discourse functions This grammar is an attempt to sketch, in as much detail as possible, the phonology and morphology of the language, as well as to outline its syntax and semantics. In order to provide a description of this complex language with as much clarity as possible, a theoretical orientation has been eschewed, and many examples have been introduced