This paper presents the results of a comparative investigation into the learning styles of successful and unsuccessful language learners. Subjects of the study were seventeen graduate university students at Yarmouk University, Jordan. They were categorized as "successful" or "unsuccessful" learners, on the basis of their final scores on their English examination administered at the end of the semester, and their oral presentations which they had to give during the semester to measure their ability of speaking, discussing and defending themselves in English appropriately. The subjects of the study completed a questionnaire through which data were collected on their learning styles and preferences as well as their patterns of language practice and use. Findings of the study showed that there were no significant relationship between the subjects' learning styles and their proficiency or achievement in English. The study also revealed some key differences in the subjects' learning styles.