This study investigated the contexts for "optimal" use of first language (L1) in second language (L2) one-on-one speaking practice sessions in a study abroad program where English was the L1 and Arabic was the L2 of students, and the opposite applied to their speaking partners. Drawing on insights from sociocultural theory to investigate data from 17 audiorecorded speaking sessions, the study shows that judicious use of students' L1 for certain purposes does indeed work as one of several interactional resources that contribute to enhancing the development of the L2 by operating as a mediating tool to cultivate communication and L2 learning. The argument for L1 optimal use is demonstrated in two particular contexts: (1) during speaking sessions to ask for or provide linguistic assistance, and (2) at the periphery of these speaking sessions for various goals, such as showing solidarity, warming up, and agreeing on what to discuss and how to proceed during interaction. Pedagogical implications are discussed.