Abstract : Naxi, Na and Laze are three languages whose position within Sino-Tibetan is controversial. We propose that these languages are descended from a common ancestor ("proto-Naish"). Unlike conservative languages of the family, such as Rgyalrong and Tibetan, which have consonant clusters and final consonants, Naxi, Na and Laze share a simple syllabic structure (consonant+glide+vowel+tone) due to phonological erosion. This raises the issue of how the regular phonological correspondences between these three languages should be interpreted, and which phonological structure should be reconstructed for proto-Naish. The regularities revealed by the comparison of the three languages are interpreted in light of potentially cognate forms in conservative languages. This comparison brings out numerous cases of phonetic conditioning of the vowel by the place of articulation of a preceding consonant or consonant cluster. Overall, these findings warrant a relatively optimistic conclusion concerning the feasibility of unraveling the phonological history of highly eroded language subgroups within Sino-Tibetan.