African township schools are characterised by cultural and linguistic diversity, hence, teachers have the dual task of ensuring that learners grasp scientific concepts, while also catering for the diversity in the learners' backgrounds. The study reported on here was aimed at investigating teachers' beliefs and practices in teaching life sciences using English, a language that is not their own home language. The study was underpinned by a socio-constructivist perspective, emphasising how one's personal context, including prior experiences, influences the development of beliefs about language use in life sciences classes and the manner in which teaching and learning might occur. The sample comprised 6 teachers who all spoke English as a second language. We collected the data using structured interviews to ascertain the teachers' beliefs about the teaching of life sciences in English to Grade 11 classes, and classroom observations to identify their classroom practices. The findings indicate that the beliefs that the teachers expressed differed from their actual choices and practices in the lessons observed.