In this study, the effect of wastewater composition on the activated sludge microflora has been evaluated. Wastewater samples from the aeration tank of a petrochemical industry wastewater treatment plant were taken and analyzed during a four-months period. The results of analysis indicate that the density of floc-forming, microorganims was higher than that of filamentous ones in the activated sludge tank, when sludge volume index (SVI) was lower than 150 ml/g. During the study, pH values of the mixed liquor were very high for fungal growth. Consequently, lower amounts of fungal species than floc-forming microorganisms were observed. Increasing, protozoan concentration in activated sludge process (ASP) caused lower amounts of suspended solids (SS), washed out especially at low dissolved oxygen (DO) concentration. Food/microorganisms (F/M) ratios were varying between 0.3-0.7 g BOD5/g mixed liqior suspended solids (MLSS) in the process. Even when influent wastewater contained low nitrogen and phosphorus levels, high BOD5 removal rates were observed, provided by filamentous microorganisms dominating in the unit. When the organic loading was higher than 14 g/m(3) /d, influent COD concentrations were higher than 2100 mg/L, and thus Pseudomonas spp. in the ASP were affected negatively.