The purpose of this study was to explore the relationship between learners' self-regulated language learning and proficiency and to examine the differences in characteristics of self-regulated learning (SRL) between low- and high-proficiency learners. SRL is a learning process throughout setting goals, monitoring tasks, and reflecting on performance, which includes both cognitive and affective aspects. Participants were 97 Japanese university students of English. Based on their proficiency, 67 out of them were assigned to the L group and 30 to the H group in order to compare the difference of their SRL skills. They all took the Motivated Strategies for Learning Questionnaire (MSLQ) online to measure their SRL skills and reported their latest TOEIC scores to determine their English proficiency. The MSLQ includes 81 items that were administered on the website for ease of access. Exploratory factor analyses determined five motivational and six learning strategy factors. Based on these factors, multiple regressions and t-tests were performed. Multiple regressions examining the influences of SRLs on proficiency subsequently showed that three learning strategy factors in SRL--metacognitive strategies, effort regulation, and coping with problems--significantly predicted the variance in learners' proficiency; no motivational factors predicted it, even though correlations between three out of five factors and proficiency were found, namely, self-efficacy, intrinsic goal orientation, and test anxiety. Later t-tests, however, showed significant differences in SRL between low- and high-proficiency learners in the following motivational and learning strategy factors: self-efficacy, intrinsic goal orientation, test anxiety, metacognitive strategies, effort regulation, and coping with problems. The findings suggest that although they did not directly account for learner proficiency, yet motivational factors were evidently related to English proficiency level. Based on these results, the characteristics of less proficient learners compared to those of more proficient learners are discussed.