In this thesis, we study the phonology and the verbal morphology of Japhug, a Sino-Tibetan language of the rGyalrongic subfamily, spoken in rNga-ba district, Sichuan Province, China. Firstly, we analyse the synchronic phonological system, including the structure of the syllable, using morphophonological alternances. Secondly, we study Japhug phonology from a diachronic point of view, proposing a set of criteria to distinguish loanwords from cognates between Japhug and Tibetan. We also propose to reconstruct some Japhug phonological changes on the basis of comparison with other languages of the rGyalrongic subfamily. Thirdly, we discuss verbal morphology, including flexional morphology, derivational morphology and nominalization. The languages of the rGyalrongic subfamily are extremely archaic, both regarding morphology and phonology. These languages preserve many characteristics of the proto-language that remain only as traces in Tibetan or in Chinese. In spite of the remarkable interest of these languages for Sino-Tibetan comparative grammar, they remain poorly described up to now. This work is the first monograph devoted to the Japhug language.