In Mauritius, English, the least socially used language, is the main language of literacy and the main written medium of instruction throughout the education system, starting from the first year of compulsory primary education. The importance of English as a school language is reflected in the 2003 Preschool Curriculum Guidelines, which mention the need for preschools to introduce children to English in preparation for compulsory primary schooling in English. The development of oral English proficiency as a basis for literacy instruction and education in English is in line with the extant evidence-based reading research which shows that oral language proficiency, often investigated and tested through receptive vocabulary, is a stepping stone to early literacy development. The aim of this study is to test the receptive English vocabulary of a group of preschoolers at the beginning and end of their final preschool year, in order to assess their developing English language proficiency. To do so, a contextually relevant test was designed and administered to a group of 4-year-olds using a pre-test/post-test design. The results indicate that during their final preschool year, preschoolers develop minimal English receptive vocabulary skills, characterized mainly by their knowledge of English number names and related number concepts, body parts and marginal familiarity with general English vocabulary items. This negligible and slow progress in English receptive vocabulary development is discussed in relation to the quantity and quality of comprehensible input being provided in the preschool classroom, as revealed by the data collected from a longitudinal study using ethnographic data collection tools. The paper ends with pedagogical implications for preschool education in Mauritius and other similar contexts.