Heterotrophic Biofilms, growing in the successive three treatment stages of the RBC reactor in a full-scale wastewater treatment plant in the summer of 2002, were investigated and compared to those growing in the fall of 2001. The seasonal changes in biofilm structure were studied in terms of biofilm thickness, dry solid density, internal pore distribution and patterns, and surface roughness. The comparison results indicated that due to the higher wastewater flow rate in the summer of 2002, these biofilms had thinner, denser and less rugged surface structure than the biofilms sampled in the fall of 2001; furthermore, they showed less heterogeneous internal pore distribution. However, biofilms from both seasons showed some similarities: biofilms in the treatment stages 2 and 3 had similar structure; and their internal pore distribution and pore surface roughness didn't show any correlation with the biofilm depth.