While bilingual children are said to differentiate their two languages as early as the babbling stage, cross-linguistic influence is commonly found in bilingual first-language acquisition, and usually this influence is of a negative sort, either delaying or interfering with the acquisition of target forms. Less commonly acknowledged is that bilingualism can also accelerate first language acquisition of syntax; this study of spontaneous child speech suggests that the development of syntactic structure itself in one of a bilingual child's two grammars can facilitate access to that structure in the child's other grammar. In particular, I examine the development of the C-domain—site of the syntax-pragmatics interface and predicted to be susceptible to cross-linguistic interference—in three Spanish-English bilingual children (ages 1;3 to 3;3), analyzing all wh-question productions in child and adult speech for word order, verbal inflection, and information structure. Contrary to expectations, English-Spanish bilingual children produce virtually perfect wh-questions in both languages from the earliest stages of development, distinguishing them from English monolinguals, who make frequent errors of auxiliary omission as late as age 3. To account for this performance, I propose the Structural Transfer Hypothesis, which explains that facilitation occurs when there are grounds for syntactic bootstrapping; delay occurs when discourse-pragmatic constraints conflict; and interference occurs when grammar-internal constraints are confounded. The application of this hypothesis to the Spanish-English pair requires an argument for early structure-building on the basis of the contents of child-directed speech in Spanish, and this is presented along with several retrospective analyses of other instances of cross-linguistic influence attested in the literature.