by a young child, for whom Irish is a second language, English her first. The data on which it is based was recorded by her mother at home over a period of one year - February 1984-March 1985. The child was exposed to the second language in an Irish-medium playgroup (Naionra) for two years and subsequently attended an Irish-medium national school. The home language is mainly English but Irish is spoken in a limited number of contexts and with certain people. The analysis shows how language emerges from interaction but highlights mainly the child's contribution. Eithne gradually built up for herself the main structures of the language, from single word to complex sentences. Particularly impressive is the way in which she single-handedly sorted out the basic tense system, discovering for herself the exact match between form and function appropriate to the target.