The aim of this exploratory study was to examine the role of the First Language First model for preschool bilingual education in the development of vocabulary depth. The languages studied were Russian (L1) and Hebrew (L2) among bilingual children aged 4-5 years in Israel. According to this model, the children's first language of educational instruction (up to age three) is the language to which they are exposed at home. Their second language -- the dominant language of the country that is spoken by the majority of the population -- is acquired sequentially, after relative maturity in the first language. Fifty-one Russian/Hebrew speaking bilinguals (around age four) were selected from bilingual (Russian/ Hebrew) preschools, which used the First Language First approach, and monolingual (Hebrew) preschools. The research was designed as a longitudinal study, as the children's vocabulary was measured at two time-points -- near the beginning and the end of the academic year. The children's vocabulary was measured in both languages by examining its depth dimensions (paradigmatic semantic relations and syntagmatic semantic relations). The results demonstrated that the later immersion in L2 and continuing development of L1 within the First Language First model does not results in retardation in development of bilingual children in L2 in comparison to their bilingual peers from the monolingual (Hebrew) preschools. In addition, this model of early bilingual development enhances the linguistic interdependence of depth of vocabulary knowledge, and, therefore, supports balanced bilingual development. (author abstract)