This paper analyzes nonlinear dynamics and stability in a network of fixed-speed induction generators. A typical example of the network appears in windfarms that have recently attracted a lot of interest as an alternative renewable energy resource with large capacity. In this paper, we derive a mathematical model representing nonlinear dynamics occurring in the induction generator network and then present a phenomenon of nonlinear instability. The dynamical mechanism of the phenomenon is discussed in terms of collective variables and mode decomposition.