The effective use of Information and Communication Technology (ICT) can have positive effects on the development of learners' English abilities. To what degree it is effective is partly determined by learners' characteristics in ICT use. However, these characteristics have not yet been sufficiently discussed in Japan. This study, then, explored how the characteristics of Japanese EFL university students related to their actual use of an online English course and whether it led to the development of their English abilities. In the survey, 130 Japanese university students were asked to self-evaluate their attitudes toward computer-assisted language learning (CALL) and the use of technology in an out-of-classroom situation. As a result, it became clear that most of the students were not confident in using the technology and did not use it actively outside the classroom. Cluster analysis was employed with a focus on individual differences, revealing that the time students actually spent on the course and their high evaluations of the effectiveness of CALL did not necessarily predict development of English abilities. It was suggested that individual differences should be carefully considered in adopting online English courses effectively in higher education institutions.