Background: The educational context in South Africa does not always support the development of bilingualism to enable the cognitive and social advantages of dual language proficiency. The emphasis on English as the language of instruction overlooks bilingual competence. This study attempts to show that by considering conceptual vocabulary, professionals can gain a better understanding of childhood bilingualism.Aim: The aim of the study was to assess the receptive, expressive and conceptual vocabulary of English-isiZulu and English-Afrikaans bilingual children.Setting: The study was conducted at two primary schools in Johannesburg, Gauteng, in which the language of teaching and learning is English from Grade 1.Methods: The study employed a comparative within- and between-group quantitative design. The vocabulary of 30 Grade 1 Afrikaans-English and 30 Grade 2 IsiZulu-English bilinguals and their age-matched monolingual peers was assessed using the English Receptive and Expressive One Word Picture Vocabulary tests and adapted isiZulu and Afrikaans versions of the tests for the bilingual groups.Results: Both bilingual groups obtained significantly higher scores in English than in their home languages across receptive and expressive modalities. The conceptual scores were significantly higher than scores on single language tests. The isiZulu-English group obtained significantly lower scores than the monolingual group in English, even with conceptual scoring, while the Afrikaans-English bilingual group obtained scores similar to and even higher than those of their monolingual peers using conceptual scoring.Conclusion: This study confirms the value of considering dual language knowledge in bilingual children and underlines the need to support, develop and maintain all South African languages in education.