A combined process consisting of ultrasonic irradiation and stepwise addition of inorganic salts was proposed in order to develop an efficient pretreatment to upgrade the settling performance of activated sludge. The addition of the divalent cation Ca2+ followed by the addition of the trivalent cation Fe3+ induced the formation of extremely large flocs, which led to a significant increase in the rate of batch gravity settling. Comparison with the result obtained when the salts were added in reversed order revealed that the prior addition of Ca(2)(+ )is essential to the improvement of settling properties. Among various Fe3+ dosages following the Ca2+ dosage of 10mM, the Fe3+ dosage of 3mM showed the maximum initial settling rate of 21.0 mm/min, which was 350 times the value obtained for raw sludge. Moreover, this Fe3+ dosage was equally effective in improving the settling performance when the Ca2+ dosage was significantly reduced from 10mM to the extremely low value of 03 mM. When the effect of the stepwise addition of Ca2+ and Fe3+ was examined for the supernatant and for the consolidated sediment separated from the ultrasonicated sludge by centrifugal sedimentation, it was found that the marked flocculation effect of stepwise addition of Ca2+ and Fe3+ was attributed not to intracellular polymer substances released into the bulk solution from cells disrupted by ultrasonication but to disrupted cells themselves.