This study sought to explore teachers' experiences in teaching Mathematics and Science through second language in Standard 4 at rural primary schools in Lesotho. In addressing this purpose, a qualitative case study method of data production was used. The participants were four Standard 4 Mathematics and Science teachers from three selected schools. Qualitative data were collected through individual semi-structured interviews and lesson observations in the classroom. These were analyzed using a qualitative thematic approach. The study is informed by Vygotsky‟s social constructivist theory and Shulman‟s pedagogical content knowledge theory. The findings revealed that teachers experienced several challenges in teaching Mathematics and Science through second language in a transitional class. The greatest challenge that the participants experienced was the language that is used for instruction in Standard 4. According to the Educational Policy of the Government of Lesotho, English has been approved as the medium of instruction (MOI) from Standard 4 upwards. In Standards 1-3 pupils are taught in their mother tongue (Sesotho); thereafter the MOI changes to English. The participants reported that the shift from Sesotho to English posed a serious problem in the teaching and learning of Mathematics and Science in Standard 4. They further claimed that most pupils in Standard 4 do not proceed to the next class, since these two subjects are core subjects and the MOI makes passing them problematic. Teachers regularly employed code-switching as a strategy to make meaning for their pupils in their classrooms. It is recommended that the Government of Lesotho should consider changing the medium of instruction to English from Standard 1. It is also recommended that the inspectorate service should be decentralised and that one teacher at primary level should not teach all the subjects but teach according to specialisation. There should also be more regular in-service training of Mathematics and Science teachers. / Thesis (M.Ed.) - University of KwaZulu-Natal, Edgewood, 2011.