This paper reports findings from the development of Language Supportive Biology Textbook (LSBT) for Form I students in Tanzanian secondary schools to address the challenge of language barrier to learning. The LSBT was thus developed to facilitate smooth transition from Kiswahili to English language medium of instruction in learning Biology. The study used a participatory action research design that involved 2 experts from Tanzania Institute of Education (TIE), 2 Biology specialists from universities, 1 language specialist from UDOM, a total of 12 Biology teachers and Form I students from 12 rural secondary schools located in Dodoma, Lindi and Morogoro Regions in Tanzania. Accordingly, we designed and prototyped a Biology textbook that is language accessible, supportive and relevant to Tanzanian context. We presented Biology content using a variety of illustrations and Original Research Articlehands-on activities. The language features included English-Kiswahili glossary on each page and bilingual activities within which Kiswahili is used as a resource to learn English. Besides, the material has learning activities filled with examples drawn from the students' context. The study used classroom observations during prototyping process, analysis of experts' reviews, and the interviews with teachers and students to gather data to document the development and effectiveness of the LSBT. The overall findings indicated that students' interaction in learning through talking, negotiating meaning, forming sentences, and presenting biological facts increased than it was before. The paper recommends that it is necessary to use of action research that involves educational stakeholders to develop potential features of a language and pedagogical supportive textbooks for quality education.