This study describes the grammar of the Idaacha dialect of Yoruba in the areas ofphonology and morphophonemics within the framework of generative phonology and the autosegmental approach. In particular, it builds upon Kouyomou's (1986) major work, Phonologie de la langue Idaasha, and argues that the language has eighteen consonants, /b/,/m/, /f/, /t/, /d/, /s/, /n/, /1/, /r/, /c/, /j/, /j/, /k/, /g/, /kp/, /gb/, /w/, /h/, and twelve vowels, /i/, /u/, /e/, /o/, /c/, /o/, /a/, r/, /u/, /E /, /o/, /a/.Particular attention is paid to vowel harmony (VH) and tones. It is argued that the high vowels, /i/, r/, /u/, and /u/ do not participate in this process. As one would expect, VH rules do not apply across word boundaries; they apply before processes such as contraction, abbreviation, and compounding. In regard to tones, it is pointed out that a three-way tonal system is a major characteristic of the language. However, a phenomenon of M/L neutralization is underlined in a specific environment: a final low tone in a verb followed by a direct object noun. In this environment, a low (L) tone changes to mid (M), but the M/L alternation is optional before an initial low-toned noun.In addition to describing these phonological processes, this work examines the morphophonemics of the language. It argues that, like Standard Yoruba (SY), Idaacha hasopen syllables: V and CV. Therefore, words are shaped as VCV, CVCV, VCVCV, and longer lexical items build upon these basic sequences. Morphemes are described with special reference to derivational processes. The issue of prefixation is discussed, and it is claimed that, besides the existing nominalizing prefixes, one cannot prove convincingly, on the basis of synchronic analysis, that the initial vowel in every VCV noun is a prefix. The morphophonemics of nominals is described with regard to associative constructions, noun compounding, verb-noun contraction, and deverbal nouns. / Department of English