Research Findings: Are family literacy activities linked to gains in preschool-aged dual language learners' (DLLs') societal language vocabulary? To understand connections between literacy activities and vocabulary, we separately considered literacy activities in the respective heritage language and in the societal language, German, and accounted for heterogeneity in DLLs' family and preschool language environment. DLLs (n = 115) were administered the German PPVT-4 twice (at 40 and 51 months). Literacy activities as well as other family language environment variables were reported by mothers. Classroom language stimulation was observed with the CLASS Pre-K language modeling scale. Two different models of change revealed that family literacy activities in the heritage language predicted gains in German vocabulary for children with low levels of classroom language stimulation (albeit at the p = 0.10-level in one set of models). There was no evidence that literacy activities in German were positively connected to gains in German vocabulary. Practice or Policy: This study suggests that family literacy activities in the heritage language may have the potential to support societal language vocabulary. More research is needed to understand under what circumstances family literacy activities in the societal language are supportive of DLLs' societal language vocabulary.