This study investigated some physical properties, and morphological and internal structural features of growing biofilms on suspended moving media in a pilot hybrid reactor. The studied biofilm physical properties were thickness and density; the structural features were boundary/surface roughness, porosity, Sierpinski Fractal Dimensions (S.F) and pore boundary fractal dimensions. This study was undertaken at both low and high flow loading conditions, and as a follow-up of the previous study done by Xiao (2004) that characterized the matured biofilms' structural features under the same loading conditions in the same hybrid reactor.Comparisons of the experimental results from both studies show that the growing biofilms had less compact structural features (higher porosity) and less rugged surface than the matured biofilms. The growing biofilms had more heterogeneous internal pore size distributions than those of the matured ones. The pore boundary roughness (Db) of the growing biofilms showed increased heterogeneity and irregularity with the increase in the biofilm growth to a peak value, then, decreased with time to some degree of uniformity when the biofilms matured.