This paper discusses the question of whether, in a multilingual society, the child's first or second language is best as a language of instruction in primary school. Reviewing eight case studies from seven countries in which initial primary schooling was given either in the second or first language, the discussion compares achievement in reading and language in both languages and describes achievement in content subjects, especially arithmetic. Case studies from the Philippines, Ireland, and Canada report the use of a second language as the language of instruction, while studies from Mexico, Nigeria, Sweden, and the United States report use of the first language. It is concluded that, depending on the interrelated characteristics of each situation, answers must be found on a case-by-case basis. Such characteristics include the linguistic and cognitive development of the child in his or her first language, the attitudes of parents toward the language chosen for the school, and the status of the languages in the wider community. In terms of these characteristics, situations are described in which the best choice for initial instruction might be either the child's first or second language. These descriptions are followed by a discussion of issues raised in a paper (1980) devoted to education sector policy and by a series of recommendations for World Bank staff. (Author/RH)