Metasyntactic ability, which is the ability to reflect upon, and intentionally control and manipulate syntactic aspects of language (Gombert, 1988), is believed to play an important role in language skills development among native speakers and heritage language learners. Although a substantial amount of research has described the role of metasyntactic ability in written language development, little is known about the underlying cognitive processes supporting it. Metasyntactic ability requires storage, control and manipulation of information. Thus, it is thought to tap into memory capacities, namely phonological memory capacity and working memory capacity. While some studies have looked into the relationship between each of these memories and metasyntactic ability, none has examined the relationship and contribution of both to metasyntactic ability. In the present study, we investigated this among 25 heritage language speakers of Arabic, learners of French (mean age = 9 years), living in a French-speaking environment. Three measurement instruments were used: a repetition of French asemantic ungrammatical sentences measuring metasyntactic ability; a numerical complex span task to measure the working memory capacity (Oakhill, Yuill, & Garnham, 2011); and a non-word recognition task to measure phonological memory capacity. Results from the multiple regressions revealed a significant unique contribution of working memory capacity to metasyntactic ability, providing support for the importance of the control system for metasyntactic ability among heritage language learners. [ABSTRACT FROM AUTHOR]