This paper presents an experimental investigation of the hydrodynamic instabilities inside a self-excited fluidized bed. Pressure fluctuation and bed height measurements have been reported. In the slugging regime (above a critical gas flow rate), the bulk is found to exhibit a regular and periodic macroscopic pattern (large numbers of particles moving collectively in a seemingly organized manner). This state is characterized by nearly signoidal pressure fluctuations and regular bed height oscillations. As the gas flow increases the bulk motion tends toward large oscillations with intermittent smaller ones. The pressure fluctuations above the bed surface and the bed height oscillations are found to be intimately correlated. We report experimental evidence that the pressure fluctuations observed in a fluidized bed under and above its surface are caused by the oscillations of the bed height. The pressure wave initiates at the surface of the bed and propagates both upward and downward. ©1997 American Institute of Physics.