The extent to which syntactic co-activation occurs during natural reading is currently unknown. Here, we measured the eye movements of Welsh-English bilinguals and English monolinguals as they read English sentences. Target words were manipulated to create nonwords that were consistent or inconsistent with the rules of Welsh soft mutation (a morphosyntactic process that alters the initial consonant of words). Nonwords were only visible in parafoveal preview, and a direct fixation triggered the presentation of the normal English word. Linear mixed effects analyses revealed a robust parafoveal preview benefit for identity (television) compared with mutated (delevision) and aberrant previews (belevision), and a parafoveal-on-foveal effect in our bilingual sample. Bilingual readers' sentence reanalysis was affected by the implicit Welsh mutation, but only in contexts that would elicit a mutation in Welsh. Our findings suggest that morphosyntactic rules are co-activated during natural reading, however further investigations must evaluate the robustness of this effect.