Teamwork between humans and computer agents has become increasingly prevalent. This paper presents a behavioral study of fairness and trust in a heterogeneous setting comprising both computer agents and human participants. It investigates people’s choice of teammates and their commitment to their teams in a dynamic environment in which actions occur at a fast pace and decisions are made within tightly constrained time frames, under conditions of uncertainty and partial information. In this setting, participants could form teams by negotiating over the division of a reward for the successful completion of a group task. Participants could also choose to defect from their existing teams in order to join or create other teams. Results show that when people form teams, they offer significantly less reward to agents than they offer to people. The most significant factor affecting people’s decisions whether to defect from their existing teams is the extent to which they had successful previous interactions with other team members. Also, there is no significant difference in people’s rate of defection from agent-led teams as compared to their defection from human-led teams. These results are significant for agent designers and behavioral researchers who study human–agent interactions.