Unlike English, where plural marking is obligatory on all (plural) count nouns, the use of the Korean plural marker -tul is constrained by specific discourse/pragmatic- and syntax-related factors. -Tul is optional on bare nouns, but preferred with human nouns, obligatory on nouns that are specific (e.g., have demonstratives), and disallowed on classifiers. The use of this marker by heritage language speakers has not been investigated before. The present study examines the usage of Korean -tul by adult heritage language speakers who are also simultaneous bilingual speakers who were exposed to both Korean and English from birth. The results of paper-and-pencil elicited production and acceptability judgment tasks indicate a disparity in the speakers' production and interpretation of -tul, as well as in their relative mastery of the discourse/pragmatic- and syntax-related factors constraining the appropriateness of -tul in various contexts.