English is a language of international communication hence it has special place in educational systems of most countries in the world. In Kenya, English is a language of instruction as well as examinable subject in Kenyan schools. In addition, Kiswahili is a national language which is used by both learners and teachers leading to code-switching. The purpose of the paper was to establish the extent of code-switching in English and Kiswahili during English lessons in public primary schools. The study adopted descriptive survey design and mixed methods approach. The target population was 243 teachers of English, 64 grade three teachers and 2176 learners in primary schools in Eldoret West Sub-County, Kenya. The sample size was 33 teachers of English, 8 grade three teachers and 292 learners. Both quantitative and qualitative data was collected through the use of questionnaires, interviews and observation schedule. Piloting of research instruments was conducted in the nearby Wareng Sub County which shares similar characteristics as the study area. Validity was ensured through expert judgment while reliability was tested using Cronbachs alpha coefficient after piloting. The data was analyzed using both qualitative and quantitative techniques. Quantitative data was analysed using descriptive statistics frequencies and percentages while content analysis was used to analyze qualitative data. The study found out that majority (56.5%) of teachers used code switching most of the time while explaining meaning of words and most (50.4%) of the teachers of English used code switching as a way of explaining grammar in an explicit manner while most (41.2%) of the teachers used code switching to introduce unfamiliar topics to learners during English lessons. The findings will benefit the Government of Kenya in formulating language policies for schools. The study will also be of significance to teachers of English in coming up with the strategies to boost performance of English.