Mathematics education researchers (MERs) use practices unique to the mathematics education discipline to conduct their work. MERs' practices, i.e., ways of being, interacting, and operating, define the field of mathematics education, are initially learned in doctoral preparation programs, and are encouraged and sanctioned by conferences and publications. Disciplinary practices facilitate MERs' interactions within mathematics education. When working in interdisciplinary groups, differences in disciplinary ways of being, interacting, and operating can create challenges with completing research and other work. Since MERs' engagement in interdisciplinary collaborations is encouraged and can result in products contributing to the evolution of the mathematics education discipline, it is important to explore what practices MERs use in interdisciplinary collaborations. We interviewed four MERs who led international interdisciplinary collaborations and used qualitative content analysis to create descriptions of practices described by MERs in their collaborations. Five practices were common between the MERs in interdisciplinary collaborations. MERs conducted interdisciplinary work by using practices that allowed them to situate themselves and others in the group (i.e., being practices), develop ideas (i.e., interacting practices), work towards common goals, and use structures to get the work done (i.e., operating practices). We argue that MERs developed new practices to position themselves and others, interact with practitioners from other disciplines, and get interdisciplinary work done. This study contributes to the evolution of the mathematics education discipline by offering five practices that can orient MERs to conducting interdisciplinary work and discussing how MERs experience interdisciplinary collaborations beyond providing mathematics education expertise.