The linguistic interdependence hypothesis (Cummins, 1979, 2000) states that children's second-language (L2) proficiency is, to some extent, a function of their first-language (L1) competence. Previous studies have examined this hypothesis with focus on a unidirectional relation from L1 to L2. In the present study, we examined "bidirectional" influences of literacy skills in multilingual contexts, and whether the nature of relations varied as a function of literacy instruction environment. To do so, we used longitudinal data from a randomized controlled trial of a literacy intervention for children in Grades 1 and 2, learning to read in Kiswahili and English, two official languages in Kenya. Children in the treatment condition received explicit and systematic instruction on literacy (e.g., phonological awareness, phoneme-grapheme correspondences) in Kiswahili and English, whereas children in the control condition did not. Overall results supported bidirectionality of relations, such that children's literacy skills in the two languages were reciprocally related over time. However, directionality of relations differed as a function of language and literacy instruction condition, such that the relation from English to Kiswahili was found across intervention conditions, but the relation from Kiswahili to English was found only among children who had received explicit instruction in Kiswahili reading. These results are discussed in light of theory and practice for language and literacy acquisition in multilingual contexts.