Research suggests that late bilinguals may have persistent difficulties with the automatic access and use of some second language structures because of a lack of underlying integrated knowledge of those structures. In contrast, early bilinguals show advantages in aspects of language use that require this type of automatic knowledge. This study investigated whether early and late English-Spanish bilinguals evidence integrated knowledge of agreement in Spanish by examining their sensitivity to agreement errors while reading for comprehension. The results of a pilot and two experiments indicate that both early and late bilinguals do possess integrated knowledge of subject-verb number agreement and noun-adjective gender agreement in Spanish, although sensitivity to agreement errors interacts with properties of the experimental stimuli. (Contains 9 tables and 5 notes.)