Two case studies of children being brought up trilingually, (English, French and Irish) are presented. Patterns of language use are described and compared for the three languages, with the emphasis on function more than degree of proficiency. It is shown that the use of the minority language (Irish) is dependent on the exclusion of the majority language (English). The paper also looks at the validity of Lambert's (1975) distinction between 'additive' and 'subtractive' bilingualism, and his claim that the 'roots' of bilingualism are in the sociopsychological aspects of language use, especially in the relative status of the two languages, as perceived by the learner