The distinction of two types of intransitive verbs—unergatives (with underlyingsubjects) and unaccusatives (with underlying objects)—may not exist at early stages of L2acquisition, both being syntactically represented as unergatives. This idea, referred to here as theUnaccusative Trap Hypothesis, provides an elegant developmental account for a variety ofseemingly unrelated syntactic phenomena in L2 English, Japanese, and Chinese. Target languageinput, structural constraints on natural language linking rules, and linguistic properties of alearner's L1s shape stages in the reorganization of the lexical and syntactic components ofinterlanguage grammars. Although nonnative grammars may initially override the structuralconstraints postulated as the Unaccusative Hypothesis (Burzio, 1986; Perlmutter, 1978) and theUniformity of Theta Assignment Hypothesis (Baker, 1988), at later developmental stages somemay still achieve conformity with the norms of natural languages.