This article presents two experiments that examine the effects of the native language (L1) on the second language (L2) acquisition of argument structure. The linguistic focus is on agentive verbs of directed motion (march, walk) and change-of-state verbs (break, melt) in Spanish and English. Agentive verbs of directed motion undergo a transitivity alternation in English when there is a prepositional phrase (The captain marched the soldiers to the tents) but not in Spanish (*El capitán marchó a los soldados hasta el campamento). Two experiments are reported that examine whether Spanish and Turkish learners of English at the intermediate level undergeneralize the transitivity alternation with manner-of-motion verbs, and whether English learners of Spanish overgeneralize the alternation. In both experiments subjects performed a picture judgment task and a grammaticality judgment task. Results confirmed that the L1 constrains the acquisition of argument structure: There were overgeneralization errors with manner-of-motion verbs in the Spanish study and there were undergeneralization errors with these verbs in the English study. Learnability implications are discussed.