This paper reports on analysis of language skills targeted and assumed within Tanzanian curriculum documents, before and after the introduction of English as the language of learning and teaching (LoLT). In many multilingual contexts, children start their basic education in a familiar language but are later expected to transition to using a new, usually unfamiliar, LoLT. As arguments for extending the use of familiar languages beyond lower primary gain influence, it is instructive to study how curricula prepare and support learners through transition in the LoLT. In Tanzania, transition from using Kiswahili as the main LoLT to using English occurs at the beginning of lower secondary education. Lower secondary education is the point where the curriculum divides into discrete subject disciplines with their own specific academic language practices. Previous research shows that explicit teaching of academic language can reduce the linguistic challenge this creates. Analysis in this research focused on two types of documents that specify and elaborate the curriculum at the national
level: syllabi and state authorised textbooks. Language skills targeted in the subject English language within Kiswahili medium primary schools were compared with those expected in the subject biology in the first year of English medium secondary education. Findings indicate a large gap between the reading, writing, talking and listening skills targeted by the specified curriculum for English language and those that are demanded by the biology curriculum. The primary education English curriculum was poorly specified with limited attention to developing the productive skills of writing and discussion, which secondary school curriculum assumes students have mastered. The biology textbook had a high density of technical vocabulary, making it difficult to read, and did not support development of academic language skills. The gap in language skills targeted in upper primary and those expected in lower secondary creates a broken language learning journey, which may be mitigated if science teachers know how to provide scaffolding for language learning. We recommend adjusting the pace of subject curricula to take account of the linguistic challenge of introducing a new LoLT and to allow time for explicit teaching of academic language skills. We also suggest that focusing on key scientific concepts and structures, whilst reducing the number of specific examples students are expected to recall would contribute to reducing the vocabulary range of textbooks.