This study investigated an emergent interaction between word reading fluency and vocabulary knowledge in the prediction of reading comprehension among French immersion students in Grades 2 and 3. A group of 66 students were tested on measures of phonological awareness, rapid automatized naming, word reading accuracy, vocabulary, word reading fluency and reading comprehension in English and French at both time points. Hierarchical regression analyses were conducted to examine whether vocabulary and word reading fluency interact in predicting English and French reading comprehension. Regressions were constructed for each language and grade separately. Results showed that in Grade 2, word reading fluency and vocabulary contributed independently to reading comprehension, though an interaction between these variables was not observed in either language. By Grade 3, an interaction between these constructs emerged and was shown to predict reading comprehension in both English and French. Specifically, vocabulary was positively related to reading comprehension among students with moderate to high levels of fluency, while vocabulary did not uniquely contribute to reading comprehension among those who were less fluent. The emergence of an interaction in Grade 3 suggests that as students' reading skills become more proficient, reading comprehension outcomes are better explained by taking into account the interaction between reading fluency and vocabulary knowledge.