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