The present study investigated Korean L2 learners’ sensitivity to graded degrees of transitivity in English verbs. L2 learners and English monolinguals rated the grammatical acceptability of English sentences with three types of verbs, i.e., high-transitive (HT), mid-transitive (MT), and intransitive (IT) verbs. Higher acceptability ratings were obtained as transitivity bias increased, suggesting that both groups were sensitive to transitivity bias information. However, the L2 participants were more conservative than the English monolinguals in their use of verb bias information across all verb types. Whereas the performance of high-proficiency learners patterned with the English monolinguals for the HT and MT verbs, they behaved similarly to low-proficiency learners for the IT verbs. These findings suggest that although the L2 learners with higher proficiency were sensitive to verb bias information, they experienced difficulty in the use of intransitivity information. These results support our claim that the acquisition and use of unaccusative non-alternating verbs may pose difficulty for even L2 learners with high proficiency.