This paper reports on an experimental study that investigates the acquisition of Japanese unaccusative verbs by English-speaking learners. Following Levin and Rappaport Hovav (1995), it is assumed that unaccusativity is syntactically represented but semantically determined. The experiment is devised specifically to examine whether L2 learners are sensitive to syntactic and semantic properties associated with unaccusative verbs in Japanese, which contrast with the properties of unergative verbs. In particular, the experiment involved picture tasks with two structures: the takusan construction as a syntactic test and the -teiru construction as a semantic test. Overall results of the experiment show that L2 learners generally know the properties investigated; that is, that subjects of unaccusative verbs originate in object position, and semantic notions such as telicity and change of state are aspects of meaning relevant to the classification of unaccusativity in Japanese. Based on these results, it is argued that the mapping of verb arguments to syntactic positions is not random, but rule governed, for most of the L2 learners in the present study.