Cationic polyacrylamide (CPAM) can induce efficient harvest of microalgae at low dosages (0.5-2 mg/L), and the large flocs that form as a result are beneficial for later screen filtration. However, flocculation is dependent on the microalgae species, especially their extracellular organic matter (EOM). In this study, harvesting of microalgae with CPAM flocculation and screen filtration (100-mu m pore size) was compared, using four cyanobacteria species (Microcystis aeruginosa, M. wesenbergii, Cylindrospermopsis raciborskii, and Phormidium ambiguum) and three green algae species (Chlorella vulgaris, Scenedesmus quadricauda, and Pediastrum duple). The influence of EOM concentration and characteristics (i.e. charge density and molecular weight) was of particular focus in this study. All cyanobacteria were effectively flocculated with 1-2 mg/L CPAM, though the retention of C. raciborskii (78%) was significantly lower than the others (>90%). For green algae, only Ch. vulgaris could be effectively flocculated with retention > 90%, but a higher CPAM dosage (4 mg/L) was required. When mixed with EOM of other microalgae, the retention of S. quadridcauda and P. duple increased to > 78%. Successful retention was highly correlated with high molecular weight (MW) and negative charge density of the EOM. The > 100 kDa EOM fractions were responsible for effective retention of cyanobacteria and 30-100 kDa EOM fractions for Ch. vulgaris, while these fractions were rarely present in the EOM of S. quadricauda and P. duple. Though effective for cyanobacteria, the application of CPAM flocculation and screen filtration for harvesting some green algae is limited due to a lack of high-MW and negatively charged EOM components.