Health professionals' roles and scopes often overlap, creating a need for role clarity in interprofessional teamwork. Yet, such clarity does not mean roles are fixed within teams and some literature suggests role flexibility can enhance team functioning. Interprofessional practice competencies and learning activities often emphasize knowledge and definition of roles, but rarely attend to the dynamic nature of roles and influential contextual factors. This study explores role fluidity in interprofessional student groups using an activity theory framework. Using a collective instrumental case study approach, the authors examine the fluidity of one physical therapy (PT) student's role within 3 different interprofessional (medical, pharmacy, PT) student groups completing nursing home patient care plans. Field notes, group debriefing interviews, and care plans were collected and coded from all care planning sessions. Codes mapped to group-specific activity systems that compared role-influencing interactions and tensions. The PT student's role fluidity varied in each group's activity system, influenced primarily by system tensions from implicit rules (e.g., encouraging questions), division of labor (e.g., rigid profession-based task assignment), and tool use (e.g., computers). Attention to modifiable system elements, such as tool use and explicit rules of inclusivity, could foster role fluidity and improve interprofessional teamwork and learning environments.