This study explores the extent to which bilingual speakers in stable bilingual communities become fully bilingual in their two community languages. Growing evidence shows that in bilingual communities in which one language is very dominant, acquisition of the dominant language may be quite unproblematic across sub-groups, while acquisition of the minority language can be hampered under conditions of reduced input. In Wales, children are exposed to both English and Welsh from an early age, either in the home or at school, or both. The data reported here indicate that regardless of home language background, speakers develop equivalent, mature command of English, but that command of Welsh is directly correlated with the level of input in Welsh in the home and at school. Furthermore, maintenance of Welsh in adulthood may be contingent on continued exposure to the language. The data have implications for theories of bilingual acquisition in stable versus immigrant bilingual communities, for optimal conditions for bringing up bilingual children, and for theories of critical periods of acquisition.