An important problem for a language learner is identifying how properties of argument structure are realized morphosyntactically in the particular language they are learning. Speakers of some L1s overgeneralize the morphosyntactic reflexes of the movement of Theme objects in English to unaccusative intransitive verbs, using passive morphology in such cases. In contrast, it appears that the same L2 learners are less likely to do this with unergative verbs. The present paper addresses the question of whether L2 learners of English with typologically different L1s such as Japanese and Spanish will overgeneralize passive morphology to unaccusative verbs in the same way. The results show that both Japanese and Spanish learners of English differentiate unaccusatives from unergatives. However, the two groups differ in their treatment of unaccusatives with transitive alternants and intransitive alternants, suggesting influence of the morphological properties of the L1.