This experimental study on English, Spanish, and Turkish as second languages investigates the interaction of universal principles and L1 knowledge in interlanguage grammars by focusing on verbs that participate in the causative/inchoative alternation (such as break in English). These verbs have the same lexico-semantic composition, but differ crosslinguistically as to how they encode the alternation morphologically. Results of a picture judgment task show that, as in L1 acquisition, L2 learners of Turkish, Spanish, and English with different L1s rely on a universal mechanism when learning transitivity alternations. L1 influence plays a prominent role in the morphological realization of the alternation. These findings suggest that UG and L1 knowledge may not affect all linguistic domains in the same way at a given stage of development. It is proposed that transfer is subject to modularity in interlanguage grammars.