In this paper, we report a study of the dynamic response of an erbium-doped fiber ring laser. When the pump light is sinusoidally modulated with small modulation index, the lasing light displays a resonant peak at a particular modulation frequency. However, as the modulation index was increased, two regions of bistability were observed. Furthermore, by fixing the modulation frequency but varying the modulation index, we also observed another region of bistability. A simple theoretical model was developed which produced results in good agreement with the experimental ones.