This study examined Schachter`s argument (1974) on ESL learners` avoidance behavior likely caused by L1-L2 differences. In this study of avoidance, eight Korean graduate students carried out two types of production tasks over four sessions: oral and written production tasks in Korean as well as in English. Five native speakers of English attending an American university and five native speakers of Korean living in Korea at the time of the experiment and receiving no structured English instruction also participated in the experiment as the English control group and the Korean control group, respectively. Results showed that the experimental participants had avoided using English relative clauses. The argument of their avoidance of English relative clauses is based on the evidential data that mirrored two compelling observations: (a) the rare occurrence of English relative clauses in the contexts in which they produced quite a few Korean relative clauses, (b) the rare occurrence in the contexts in which native speakers of English produced English relative clauses. Finally, it was also observed that such relative clauses as SU and DO relative clauses, which are higher on the accessibility hierarchy (Keenan & Comrie, 1977), accounted for the overwhelming majority of the relative clauses produced in all tasks, reaffirming the reliability of the accessibility hierarchy.