Due to the lack of robust ways to quantify aggregations, fouling of two-foulant aggregations is poorly understood. This work systematically reports the ultrafiltration membrane fouling by aggregations formed from two oppositely charged organic foulants (i.e., humic acid (HA) and lysozyme (LYS)) with the aid of resonance light scattering (RLS) technique. RLS provides an effective approach to detecting the aggregation concentration and reveals that the HA-LYS aggregations were formed at a mass ratio of m(LYS)/m(HA) = 2.77. During the filtration of the mixture of HA and LYS, aggregations over individual foulants were identified to be the main substances deposited on the membrane surface, where the mass of deposition had a good linear relationship with the feed concentration of the aggregations. The HA-LYS aggregations might decrease the total fouling due to their large size, but reduce the fouling reversibility. In the pH range of 5.5-9.2, the pH value had limiting effects on the concentration of HA-LYS aggregations, as well as the consequent fouling. At low ionic strength, the membrane fouling by HA-LYS aggregations decreased as the ionic strength increased due to the reduction of the aggregation concentration. Oppositely, at high ionic strength, this tendency was reversed due to the electrical double layer compression effect. These results suggest that RLS is a simple and effective way to quantify the aggregations of foulants, and the aggregations of foulants have distinct fouling behaviors compared with the individual foulants.