Previous studies have shown that learner individual differences have important impacts on L2 accuracy development. The present study examines a learner variable (i.e. extraversion versus introversion) and its effects on incidental focus on form. Twenty-eight ESL students in two classes (an advanced and an upper-intermediate class) and their teacher participated in the study. Data were collected through classroom observation over 7 weeks (for a total of 16 hours) and background and personality trait questionnaires. All sessions were audio- and video-recorded. The classroom data were first coded for the instances of focus on form episodes (FFEs) and then individualized post-tests were created and administered to each student after the final classroom observation. The results revealed that more extraverted learners were more likely to engage in FFEs in the advanced class and that more introverted learners produced more successful uptake in the upper-intermediate class. The findings partially support the role of this learner variable in learners' participation in FFEs and its effects on immediate L2 improvement.