Membrane fouling is a major obstacle for applying Ultrafiltration (UF) in algae-laden water treatments. Both membrane permeability and energy consumption may be negatively influenced by the accumulation of algal bodies (i.e., algal cells and debris) and algae-derived organic matter, including extracellular organic matter (EOM) and intracellular organic matter (IOM), on the membrane surface. This study was aimed to investigate individual and combined UF membrane fouling by algae-derived foulants including cells, debris, EOM and IOM to identify the main foulants and to gain an increased understanding of fouling mechanisms involved in algae laden water treatments using UF. The characteristics of the algal foulants were determined with respect to particle distribution, zeta potential, interfacial free energy, and fluorescence excitation-emission matrix (FEEM) spectrum. The results indicated that algal cells resulted in the fastest flux decline during filtration, but that algal organics (i.e., EOM and IOM) caused more adsorptive and irreversible fouling than did algal cells. For the combined fouling, the flux decline was considerably aggravated because the algal organics filled the voids in the cake layer formed by the cells and debris, but the fouling reversibility was not substantially aggravated due to the retention of EOM and IOM in the aforementioned cake layer. A synergistic effect was not observed in UF membrane fouling caused by the combined foulants in this study. Furthermore, both algal debris and IOM caused severe flux decline and irreversible fouling, suggesting that cell breakage should be strictly controlled for real algae-laden water treatments that employ UF.