In this paper, we suggest a message-based communication protocol and a dynamic role assignment mechanism for efficient cooperation and teamwork of agents in computer game environments. This role assignment mechanism is a new one different from both existing static and dynamic mechanisms, in which all decisions related with role assignment are made at once in design phase or execution phase. According to our mechanism, all possible role sets are determined in design phase. Detail decisions regarding which agent takes what role, however, are made in execution phase. This mechanism for role assignment can minimize the negotiation effort in execution phase. Therefore, this mechanism is quite effective especially in real-time multiagent environments. Through experiments, we show the superiority of the new dynamic role assignment mechanism.