The investigation concerns phonemic contrasts (including tonal contrasts) that are under a severe danger of disappearing from the Na language spoken in the plain of Yongning (Ninglang prefecture, Lijiang municipality, Yunnan province, China). This language belongs to the Sino-Tibetan family; it is also known as "Mosuo" and "Yongning dialect of Eastern Naxi". The overall tendency is, unsurprisingly, that the phonemic contrasts found in Mosuo but not in Mandarin Chinese tend to become lost by the speakers who are most proficient in Chinese. We discuss several examples in detail. Among other consonants, Na contrasts /l/，/ɬ/，/ʁ/，and /ɻ/; this set is fully preserved in the speech of a 55-year-old speaker, whereas in the speech of a schoolgirl (currently in senior high school) the four have simplified to /l/: /Mli/ "to look" and /Mɬi/"ear; moon" have both simplified to /li/; the syllables /ʁæ/ and /ɻæ/ in /Mʁæ.mi/"sword" and /#Hkʰwæ.ɻæ/ "satin" all simplified to /læ/. In-between these two extremes, in the speech of a speaker aged 35, these phonemic contrasts are preserved but their lexical distribution is becoming somewhat imperfect. For instance, she pronounces the word "sword", /Mʁæ.mi/, which is not in common use anymore, as /Mɻæ.mi/; this example arguably constitutes an indication that the opposition between /ʁ/ and /ɻ/ is weakening. As far as tones are concerned, the differences between the oldest generations and the speakers under 60 years set in sharp relief the complexities of the tonal system of Na: the contrasts that are neutralized in citation form tend to be overlooked by the less proficient speakers, leading to a major transformation of the architecture of the tonal system.