Aims and objectives/purpose/research questions: This study investigated three key issues in heritage language research. Previous research shows heritage language speakers have an advantage on tasks of oral production compared to L2 speakers who instead perform better on written tasks requiring use of metalinguistic skills. Furthermore, both L2 and heritage speakers are claimed to have a yes-bias towards retaining ungrammaticality in grammaticality judgement tests (GJTs). Finally, the morphological domain has been shown to be as problematic for heritage language speakers as L2 speakers, but research in lesser-known languages is needed. Design/methodology/approach: Adult L1, L2, and heritage language speakers of Italian were compared on an oral priming task and timed GJT. Both accuracy and response times were elicited from the latter test. The forms investigated were object and si-passive pronouns which lack corresponding forms in Swedish, the dominant language of the bilingual groups. Data and analysis: Mixed-effects regression was modelled to accuracy on the priming and GJT and response time data from the GJT. In contrast, a d-prime analysis was used to measure the degree of sensitivity to grammaticality and bias towards correct and incorrect answers in the GJT. Findings/conclusions: Overall, the two bilingual groups performed quite similarly across the measures tested. All three groups show high sensitivity to grammaticality and a very similar bias for yes-answers on both grammatical and ungrammatical items. Originality: The study is the first to employ a d-prime analysis to explore in greater detail the differences in knowledge of grammaticality between heritage, L2, and monolingual populations. It also presents a brief review of the limited existing research in heritage Italian. Significance/implications: Any advantages by task for either bilingual group level out by the time high proficiency levels are reached but may be associated with literacy levels when metalinguistic skills are measured. The yes-bias is likely a characteristic intrinsic to GJTs rather than a peculiarity of bilingual speaker knowledge. Morphology is problematic also for heritage speakers of lesser-known languages. © The Author(s) 2022.