The organic matter and surfactants in wastewater may cause variations in the surface tension of wastewater (STW) ranging between similar to 40 and similar to 70 mJ center dot m(-2). This study focused on the influence of STW on microalgae biofilm formation. A theoretical analysis was first conducted, and then microalgae biofilm formation on hydrophilic and hydrophobic substrata in liquid and real wastewater with different surface tensions was studied. The results demonstrated that STW affected microalgae biofilm formation remarkably. When the surface tension of liquid medium (gamma(lv)) was approximately equal to the average value of surface free energy of microalgae and substrata, biofilm formation reached the minimum. Microalgae biofilm formation on a hydrophilic surface first decreased (from similar to 2200 to similar to 1500 cells/mm(2)) and then increased (from similar to 1500 to 3100 cells/mm(2)) with the decrease in gamma(lv) (from similar to 70 to similar to 40 mJ center dot m(-2)), whereas biofilm on a hydrophobic surface continued to decrease (from similar to 2500 to 1000 cells/mm(2)).