Research has established that reading and writing skills are closely linked for most individuals, which can be explained by the shared knowledge and functional relations theoretical perspectives. The current study used quantile regression to explore differences between and within Grades 3-5 in the United States in the associations between a measure of reading automaticity (words correct per minute; WCPM) and writing automaticity (correct minus incorrect writing sequences; CIWS), when controlling for reading comprehension. Four hundred and twenty-two students completed a writing-in-response-to-reading task. Results indicated that WCPM and CIWS were significantly associated for most students in Grades 4 and 5 and only the strongest writers in Grade 3. In Grade 5, the association between reading and writing fluency were significantly stronger for the fluent writers than for the weaker and average writers. These results indicate that performance in writing-in-response-to-reading tasks may be facilitated by automaticity in reading, and that more automatic writers will typically also be automatic readers. The results suggest that late elementary grades may be an important developmental period for the integration of reading and writing fluency and that students may benefit from instruction to support this integration.