The transmission and scanning electron microscopes were employed to visualize the sequence of the biofilm development in the trickling wastewater filter. After the deposit of a small amount of debris upon a hard surface, the bacterial cells attach and develop the matrix on which the biofilm is formed. Protozoa invade the basic layer where they feed on the bacteria. The algae are seeded upon the bacterial matrix and grow so profusely that the bacteria must develop aerial colonies in the competition for food and oxygen. Destruction of the bacteria in the matrix and the weight and hydraulic pressure cause detachment of the biofilm and a new matrix must be developed.