Meta-analytic research on the effectiveness of second language (L2) instruction has consistently shown that instruction have positive effects on L2 learning, and that different types of instruction produce differential effects on the development of L2 proficiency (Goo et al., 2015; Kang et al., 2018; Norris & Ortega, 2000; Spada & Tomita, 2010). However, it is unclear how the relative effects of L2 instruction vary according to the individual characteristics of learners, and while there is substantial evidence for the role of individual differences in L2 learning (see Li et al., 2022), less is known about the potential interaction of individual differences with educational treatment (i.e., aptitude-treatment interactions [ATIs]; Cronbach & Snow, 1977). Examining possible ATIs in L2 acquisition is important because instructional effectiveness depends largely on learners’ individual characteristics (DeKeyser, 2021). Individual differences in declarative memory and procedural memory, the two most critical learning and memory systems in the brain (Ullman, 2020), have been found to be important sources of variability of L2 learning (see Hamrick et al., 2018; Morgan-Short et al., 2022; Morgan-Short & Ullman, 2022; for reviews). Declarative memory is a memory brain system that supports conscious recollection of facts (semantic knowledge) and events (episodic knowledge) (Squire, 2004), as well as the learning of arbitrary pieces of information and their associations (Ullman, 2020). Learning in this memory system can occur rapidly and after a single exposure to a given piece of information (Morgan-Short & Ullman, 2022). Procedural memory, on the other hand, is a memory brain system responsible for motor skills, habits, and actions that are relatively implicit and difficult to verbalize (Knowlton et al., 2017); it underlies unconscious sequence processing and learning (Ullman et al., 2020). Learning in procedural memory takes place gradually and it is brought about by means of generating and testing predictions on the outcome, as well as rapid feedback on the generated predictions (Morgan-Short & Ullman, 2022). In L2, and crucial for the work that we present here, these two long-term memory systems play a critical role in the learning of L2 vocabulary and grammar (Morgan-Short et al., 2022; Morgan-Short & Ullman, 2022). More specifically, declarative memory primarily facilitates associative vocabulary learning (Morgan-Short & Ullman, 2022); it can also support the learning of grammar, especially in the early stages of acquisition (Morgan-Short et al., 2022). Conversely, procedural memory is primarily involved in the learning of grammar, particularly, in the later phases of acquisition (Hamrick et al., 2018; Morgan-Short et al., 2014). Overall, individual differences in both declarative memory and procedural memory are considered to be significant cognitive predictors of L2 development (Morgan-Short & Ullman, 2022; Ullman, 2020). Using a cross-situational learning paradigm (i.e., learning by generalizing across multiple situations; Williams & Rebuschat, 2022), in this study we first set out to examine the effect of different types of instructional conditions on the learning of L2 vocabulary and grammar. The instructional conditions vary in explicitness, from a more implicit condition where learners are exposed to an artificial language incidentally, that is, without any specific instructions to pay attention to the structure of the language (Implicit Group) (Monaghan et al., 2021; Rebuschat et al., 2021, Walker et al., 2020), to two more explicit ones where learners are either given explicit information about rules/regularities (Explicit rule presentation Group), or they are instructed to search for underlying rules (Explicit rule-search Group) (Goo et al., 2015; Kang et al., 2018; Monaghan et al., 2021; Norris & Ortega, 2000; Spada & Tomita, 2010). Our second and main goal is to determine potential ATIs in L2 acquisition by means of examining how individual differences in declarative memory and procedural memory affected the ability to acquire artificial language under different instructional conditions varying in their degree of explicitness (explicit or implicit). Findings from this laboratory-based study will shed light on how learner individual differences in declarative memory and procedural memory impact the effectiveness of instruction in the context of cross-situational learning. Using a laboratory-based approach to investigate ATIs in L2 offers the advantage of examining the process and the product of language learning in a highly controlled environment, allowing for isolating the effect of instruction on learning when this effect is moderated by learner individual differences.