This paper presents a hierarchical framework for controlling a group of biomimetic fish robots to achieve cooperative tasks. Based on our previous successful work on the design and development of a robotic fish prototype, we attempt further to investigate the cooperation in groups of these fish. Employing top-down design approach, we propose a hierarchical architecture consisting of five levels: task level, role (or mode) level, behavior level, action level and controller level, to formalize the processes from task decomposition, role assignments and control performance. Two typical cases are developed to demonstrate the feasibility of the architecture and corresponding experimental results show that high efficiency and much greater capabilities are exhibited when the fish try to cooperate.