Manegacha is one of Tibet’s approximately sixty minority languages and is spoken by several thousand Tibetans living in four villages on the northeast Tibetan Plateau. Fieldwork conducted in early 2016 has confirmed earlier anecdotal evidence that the Manegacha-speaking community is currently shifting to Tibetan. Although this shift is taking place in all four villages, it is particularly pronounced in one community, where the majority of families have ceased speaking Manegacha to their children. Why has this community ceased transmitting the language while others are, for the most part, continuing to maintain it? This presentation will examine this issue by drawing on interviews with Manegacha-speakers in which they expressed language ideologies about ethnicity, community, and appropriate language use. Members in all Manegacha-speaking communities expressed language ideologies that are both amenable and hostile towards continued intergenerational language transmission. Not only are supportive and non-supportive ideologies expressed in all communities, they are also often expressed concurrently by the same individuals. In light of these persistent contradictions, I argue that the crucial difference leading to language shift in some Manegacha-speaking communities is the existence of social mobilization; language shift occurs when a community mobilizes around ideologies that undermine the language, not simply when such ideologies are present in the community, or even when they are strongly held. I therefore argue that understanding social mobilization, and its capacity to translate ideology into policy, is crucial for understanding language maintenance and shift.