The current study replicated and extended the previous findings of content-integrated literacy intervention focusing on its effectiveness on first- and second-grade English learners' (N = 1,314) reading comprehension, writing, vocabulary knowledge, and oral proficiency. Statistically significant findings were replicated on science and social studies vocabulary knowledge (ES = 0.51 and 0.53, respectively) and argumentative writing (ES = 0.27 and 0.41, respectively). Furthermore, treatment group outperformed control group on reading (ES = 0.08) and listening comprehension (ES = 0.14). Vocabulary knowledge and oral proficiency mediated treatment effects on reading comprehension, whereas only oral proficiency mediated effects on writing. Findings replicate main effects on vocabulary knowledge and writing, while also extending previous research by highlighting mechanisms underlying improved reading comprehension and writing.