The rapid increase in the production and use of fullerene C(60) nanoparticles raise concerns about environmental risks and human health. Wastewater treatment plants are key barriers to their discharge into the environment. The aggregation behavior of aqueous suspensions of C(60) nanoparticles (nC(60)) could affect their transport, bioavailability, and removal during wastewater treatment. We tested the aggregation of nC(60) in wastewater at different values of pH, ionic strength, and dissolved organic matter (DOM). The nC(60) remained relatively stable in filtered wastewater under environmentally relevant conditions up to 24 h. But at pH 3 or at high ionic strength (>100 mM NaCl), the aggregate size increased greatly, reaching micrometer scale after only 1 h. However, the aggregation behavior varied among wastewater samples even at values of similar zeta potential, compared with that in filtered secondary effluent and aeration tank liquor, that in filtered primary effluent was obviously inhibited. This inhibition could be attributed to the steric stabilization due to the adsorption of DOM on nC(60) aggregate in addition to electrostatic stabilization. The aggregation results also suggest that membrane filtration could be improved by adjustments to pH.