The retention time distribution of liquid in a trickling bed of peat moss was studied. Two different flow regimes have been detected. This is due to the fractal-like structure of peat moss. At low velocities, the liquid flows through the aggregates of peat moss particles. In this case. the tracer is distributed evenly in all the volume of the liquid because of the small characteristic time of diffusion. When the superficial liquid velocity is high, the liquid mainly flows between the solid aggregates and only a small amount of tracer penetrates the liquid inside and between the particles forming the aggregates. The results obtained are important for modelling the process of biofiltration with peat moss, a new system for environmental protection.