An ultrasonic mixing system having a treatment chamber in which at least two separate phases can be mixed to prepare an emulsion is disclosed. Specifically, at least one phase is a dispersed phase and one phase in a continuous phase. The treatment chamber has an elongate housing through which the phases flow longitudinally from a first inlet port and a second inlet port, respectively, to an outlet port thereof. An elongate ultrasonic waveguide assembly extends within the housing and is operable at a predetermined ultrasonic frequency to ultrasonically energize the phases within the housing. An elongate ultrasonic horn of the waveguide assembly is disposed at least in part intermediate the inlet and outlet ports, and has a plurality of discrete agitating members in contact with and extending transversely outward from the horn intermediate the inlet and outlet ports in longitudinally spaced relationship with each other. The horn and agitating members are constructed and arranged for dynamic motion of the agitating members relative to the horn at the predetermined frequency and to operate in an ultrasonic cavitation mode of the agitating members corresponding to the predetermined frequency and the phases being mixed in the chamber.