Extending previous research that has examined the relationship between long-term memory and second language (L2) development with a primary focus on accuracy in L2 outcomes, the current study explores the relationship between declarative and procedural memory and accuracy and automatization during L2 practice. Adult English native speakers had learned an artificial language over two weeks (Morgan-Short, Faretta-Stutenberg, Brill-Schuetz, Carpenter & Wong, 2014), producing four sessions of practice data that had not been analyzed previously. Mixed-effects models analyses revealed that declarative memory was positively related to accuracy during comprehension practice. No other relationships were evidenced for accuracy. For automatization, measured by the coefficient of variation (Segalowitz, 2010), the model revealed a positive relationship with procedural memory that became stronger over practice for learners with higher declarative memory but weaker for learners with lower declarative memory. These results provide further insight into the role that long-term memory plays during L2 development.