This study examines relations between the first language (L1) and second language (L2) in reading and writing for 211 upper middle school Korean students learning English as a foreign language. Results using path analysis suggest that L1 ability is a significant independent predictor of L2 ability in both reading and writing, providing evidence of interdependent relations between L1 and L2 operations in receptive and productive skills. Results also indicate that L1 reading predicted L2 writing, but L1 writing did not predict L2 reading. L2 proficiency was also shown to be important. L2 proficiency made a greater contribution to L2 reading and writing operations than L1 ability did. Also, the relation between L1 and L2 writing skills but not reading skills was significantly moderated by L2 proficiency. The author discusses results in the context of existing theories about L1--L2 relations and highlights important implications for pedagogy and future research.