Contaminating microzooplankton have a great influence on the growth of microalgae. To illustrate how harmful contaminants affect the growth of Chlorella sorokiniana and to investigate the microzooplankton community composition, the succession of microzooplankton contaminating pilot-scale C. sorokiniana cultures was surveyed in four different seasons. Nineteen species of microzooplankton (including a cyst) were detected in the C. sorokiniana cultures, with the flagellate Poterioochromonas malhamensis and the amoeba Vannella sp. being the most harmful species to C. sorokiniana due to their high abundance and ability to graze C. sorokiniana, resulting in a decrease of 38% to 59% of C. sorokiniana production. Once the P. malhamensis abundance reached more than 1.0 x 10(4) ind. mL(-1) or the Vannella sp. abundance exceeded 2.0 x 10(3) ind. mL(-1), the C. sorokiniana population declined dramatically. More species and a greater abundance of microzooplankton were found in summer with the lowest algal biomass by the end of the cultivation. The occurrence of microzooplankton in the C. sorokiniana culture system showed a certain regularity. Network analysis indicated that the two microzooplankton species of P. malhamensis and Vannella sp. had positive correlations with the growth of C. sorokiniana and the occurrence of the other contaminating microzooplankton. Overall, this study provided novel insights into the microzooplankton community and their co-occurrence networks during C. sorokiniana cultivation, which will help provide reliable theoretical support for the effective control of contaminating microzooplankton.