The distinction of two types of intransitive verbs—unergatives (with underlying subjects) and unaccusatives (with underlying objects)—may not exist at early stages of L2 acquisition, both being syntactically represented as unergatives. This idea, referred to here as the Unaccusative Trap Hypothesis, provides an elegant developmental account for a variety of seemingly unrelated syntactic phenomena in L2 English, Japanese, and Chinese. Target language input, structural constraints on natural language linking rules, and linguistic properties of a learner's L1s shape stages in the reorganization of the lexical and syntactic components of interlanguage grammars. Although nonnative grammars may initially override the structural constraints postulated as the Unaccusative Hypothesis (Burzio, 1986; Perlmutter, 1978) and the Uniformity of Theta Assignment Hypothesis (Baker, 1988), at later developmental stages some may still achieve conformity with the norms of natural languages.