In the extant literature on the development of the SES-related vocabulary gap, effects of low-income status and immigrant background often overlap. This study compared vocabulary trajectories in Italian (societal language) with normative data and investigated environmental factors related to vocabulary development in 83 toddlers (49% girls) from equivalent lowincome monolingual (28) and bilingual immigrant (55) families, from 18 to 36 months. Childcare teachers assessed vocabulary every 6 months using the Italian MacArthur–Bates Communicative Development Inventory (CDI). Parents reported information about environmental factors. Results based on a simulation approach showed that, at each assessment time, the participants’ median vocabulary scores were lower than normative data from the CDI. Mixed models considering environmental factors revealed that bilinguals’ vocabulary in Italian was lower than monolinguals’ from 18 to 30 but not at 36 months. In both groups, maternal education and home language activities positively predicted vocabulary from 18 to 36 months.
