A series of artificial language (AL) learning studies investigated the development of shared syntactic representations during early stages of second language (L2) acquisition. In this paradigm, the sharing of syntax is measured by means of structural priming: if structures are shared between two languages, a sentence in one language (e.g., the AL) should prime the structure of a sentence in another language (e.g., Dutch) and vice versa. According to Hartsuiker and Bernolet’s (2017) developmental theory, syntactic representations evolve gradually from item-specific to more abstract with increasing L2 proficiency. The current study tested this hypothesis by focusing on individual differences in AL proficiency during the first day of acquisition in a re-analysis of three AL studies. We predicted that individuals with higher AL proficiency levels show more cross-linguistic priming than those with lower levels. AL proficiency was indeed related to the magnitude of structural priming, although the strongest evidence for modulation of priming by proficiency was obtained for item-specific priming. Additionally, we observed that working memory (WM) capacity and L1 proficiency predicted AL proficiency and priming in general. Finally, WM capacity predicted the magnitude of priming in ditransitive sentences, but not in transitive sentences, suggesting a larger role for WM in ditransitive vs. transitive priming.