The present study investigated the interface between explicit instruction and computer-assisted language learning (CALL) in an effort to reduce plural marking errors among Japanese EFL learners. The instruction involved raising contrastive awareness of cross-linguistic and conceptual features, and was delivered via an online medium. A total of 180 students from a university in Western Japan participated in the study, in which two points of enquiry were examined: (1) the extent to which such an instructional composite would affect Japanese learners' ability to (a) recognize plural omission errors in L2 reading, and (b) produce obligatory plural forms in L2 writing; and (2) the extent to which online instruction would be sufficient to facilitate change in subsequent performance related to plural accuracy. In contrast with the control condition, the findings from the experimental condition indicated that positive learning outcomes had taken place with regard to both error recognition and plural production, and that web-based instruction was therefore effective in this instance. Theoretical reflections, practical implications, and future avenues of research are discussed.