The present study investigated second language (L2) learners' acquisition of automatic word recognition and the development of L2 orthographic representation in the mental lexicon. Participants in the study were Japanese university students enrolled in a compulsory course involving a weekly 30-minute sustained silent reading (SSR) activity with graded readers for 12 weeks. They completed the masked form-priming lexical decision task (LDT) before and after the in-class SSR activity. Results showed that participants exhibited signs of increasing automaticity of L2 word recognition (analyzed with the coefficient of variation), but could not develop their L2 orthographic representation (analyzed with the pattern of priming effects in the masked form-priming LDT). These findings suggest that automatization does not necessarily entail the development of orthographic representation, that is, the acquisition of automatic word recognition and the development of orthographic representation do not occur simultaneously. Instead, their development is asymmetrical.