This study examines whether monolingually raised Spanish speakers and Spanish heritage speakers are susceptible to similarity-based interference during the comprehension of noun-adjective gender agreement. Using eye-tracking, participants read complex noun phrases modified by predicative adjectives. The complex noun phrase included a head noun (e.g., La hija ‘the daughter’) that served as the controller of the agreement relation and a prepositional phrase that had a noun that served as the intervener (e.g., de la tía ‘of the aunt’). The intervener either matched or mismatched in gender with the head noun. The results revealed that Spanish native speakers were not susceptible to similarity-based interference. The heritage speakers were susceptible to interference in early eye-tracking measures in ungrammatical sentences, but not in grammatical sentences. Interference in this study was asymmetrical. The results will be discussed in terms of Cunnings’ (2017) hypothesis on cue-based memory retrievalin “Parsing and Working Memory in Bilingual Sentences Processing.”