Categorization is a crucial component of human cognition. Multiple systems theories suggest categories can be learned by explicit or implicit processes/systems depending on the type of category (e.g., Ashby & Valentin, 2017). Research examining the interaction between these systems found that explicit learning impairs implicit performance (Ashby & Crossley, 2010; Crossley & Ashby, 2015; Sanchez et al., 2020). The nature of this impairment remains unclear. The current study examined the effect of selective attention to rule dimensions that were either relevant or irrelevant to a later implicit categorization task to better understand how this impairment occurs. The results suggested that attention to relevant dimensions is crucial for implicit learning. Both systems can learn in parallel as long as the relevant category information is attended. This suggests the primary mechanism of implicit impairment by the explicit system may be drawing attention away from relevant information rather than rule-based strategy perseveration.