The effects of language transfer have been amply documented in second language (L2) acquisition and, to a lesser extent, in the language contact/loss literature (Cook, 2003). In both cases, the stronger and often dominant language encroaches into the structure of the less dominant language in systematic ways. But are transfer effects in these two situations comparable: is first language (L1) influence in adult L2 learners similar to L2 influence in the L1 of early bilinguals? The current study addresses this question by investigating knowledge of Spanish clitics, clitic left dislocations, and differential object marking (DOM) in 72 L2 learners and 67 Spanish heritage speakers. The contact language, English, is assumed to not instantiate these syntactic properties. Results of an oral production task and a written acceptability judgment task indicated overall advantages for the heritage speakers in some areas, but similar effects of transfer from English in the two groups. Transfer effects were less pronounced with core aspects of grammar (syntax proper in the case of clitics) than with aspects of grammar that lie at the interfaces of syntax and semantics/pragmatics, as in the case of clitic left dislocations and DOM. These findings have implications for current views on the vulnerability of certain linguistic interfaces in language development (Sorace, 2004; Serratrice et al., 2004; Tsimpli and Sorace, 2006; White, 2009) and for theories that stress the role of age in L2 acquisition and permanent transfer effects. (Contains 22 notes, 6 tables, and 7 figures.)