Kiswahili is one of the most widely used languages in East and Central Africa. In Tanzania, where it is the national language, attempts have been made to develop it so that it can be used as an efficient tool of communication in all sectors of the society, including education. This paper shows that although Kiswahili has successfully been promoted as the medium of primary and adult education, at secondary and tertiary levels of education, its position is still precarious. The notion that English and Kiswahili are in complementary distribution is rejected. It is argued that the two languages are in conflict, and that those who are in a better socio-political/economic position have more control of, and better access to, English. In such a situation the right question to ask is not in which domains English is used, butwhy it is used in such domains andwho uses it. The paper further argues that the present sociolinguistic environment makes the use of English as a viable medium unsustainable. For this reason, insistence on the use of English adversely affects the learning process. It is suggested that if Kiswahili became the medium of education at secondary school level and English was taught well as a foreign language, this would help to promote both languages without jeopardising the learning process.