Abstract: The initial formation process of mixed fouling of Pseudomonas fluorescent and CaCO₃ on the different solid surfaces in a simulated cooling water system was investigated. The mixed fouling behavior on different solid surfaces under the conditions of different CaCO₃ saturation levels and bulk velocities was examined, and the growth curves of mixed fouling were obtained. The results showed that the average mass of mixed fouling was less than that of biofouling. In the unsaturated CaCO₃ solution, the average mixed fouling mass was little; but in the saturated CaCO₃ solution, with the saturation level of CaCO₃ and bulk velocity increasing, the average mass and induction period of the mixed fouling decreased. The mixed fouling mass on non- metallic material surfaces was more than that on metal material surfaces. It was also found that CaCO₃ particles were adsorbed to the surfaces firstly, then microbes were adsorbed. The existence of CaCO₃ inhibited the combination of microbes with surfaces, on the other hand, the adsorption and growth of microbes suppressed CaCO₃ fouling development.