It is now widely accepted that lexical access is language non-selective in bilinguals. However, in the domain of syntax, it is much less clear whether such non-selectivity applies. Observational production studies have highlighted many instances of syntactic borrowing between languages in bilingual children and such borrowings tend to remain in adulthood. In the present study, we have tested for the first time whether such syntactic co-activation effects also exist in comprehension, in the case of early fluent bilinguals. We used event-related potentials in conjunction with a binary decision paradigm known to elicit modulations of the N2 component to show that Welsh-English bilingual participants are prepared to accept an adjective in post-nominal position in an English sentence despite the fact such word order is illegal in English. This effect, which was not found in English monolingual participants, can be interpreted as the consequence of Welsh grammar activation because the adjective is in post-nominal position in Welsh. Our results allow us to conclude that syntactic activation is language non-selective during reading in early bilinguals.