This experimental study on English, Spanish, and Turkish as second languages investigatesthe interaction of universal principles and L1 knowledge in interlanguage grammars by focusingon verbs that participate in the causative/inchoative alternation (such as break inEnglish). These verbs have the same lexico-semantic composition, but differ crosslinguisticallyas to how they encode the alternation morphologically. Results of a picture judgment task showthat, as in L1 acquisition, L2 learners of Turkish, Spanish, and English with different L1s rely ona universal mechanism when learning transitivity alternations. L1 influence plays a prominentrole in the morphological realization of the alternation. These findings suggest that UG and L1knowledge may not affect all linguistic domains in the same way at a given stage ofdevelopment. It is proposed that transfer is subject to modularity in interlanguage grammars.