This study reports on an experimental study that investigates Korean EFL college students" acquisition of English verbs (such as break and change) that participate in the transitivity alternation. The experiment was devised specifically to examine how the distinction of the three types of transitivity alternation verbs - unaccusatives, middles, and passives (all with non-agent subject on the surface) - is revealed in the development of learners" interlanguage. A total of 80 college students were divided into two proficiency groups and asked to perform a production task and a grammatical judgment task on the target structures. The overall results show that the learners acquire passives earlier than unaccusatives and middles. The results also reveal that with increased proficiency, the learners perform better on all the target structures regardless of the task type. The results show, however, that even higher proficiency learners are not sensitive to lexico-syntactic properties associated with unaccusatives and middles. Another finding from the results is that the major type of the learners" interlanguage error is overpassivization of unaccusatives and middles, which confirms that Korean EFL learners, like learners with other L1 backgrounds, rely on a universal mechanism of subject―agent and object―patient/theme mapping.