Slime samples from 66 rotating biological contactor (RBC) in 20 States were examined. For the filamentous bacteria, Beggiatoa, Sphaerotilus, Type 0041, Type 1701 and Nocardia were observed in decreasing order of frequency. For the protozoa, flagellates, Opercularia, Difflugia, Arcella, Paramecium and Epistylis were present in decreasing order of frequency. Organisms differed in abundance along the RBC stages. Using linear regression, Beggiatoa, Type 0041 (negative), and flagellated protozoa showed significant correlations (p< 0.05) with organic loading, while Sphaerotilus and Opercularia did not. In an attempt to establish an indicator organism system, RBC organisms were grouped based upon the organic loading preference. Beggiatoa and flagellated protozoa were grouped as the organism associated with high organic loading. Type 0041, and Arcella and Difflugia were grouped as the organism associated with moderately low organic loading. The microbial population in the last stage was investigayed in association with the effluent quality. Flagellates and Opercularia were more frequently observed in plants with effluent BOD`s above 20 mg/l, while Arcella and Difflugia were more frequent in the plants with effluent BOD`s below 20 mg/l.