Aggregation and flocculation of latex particles have been examined in an ultrasonic standing wave trap that draws microparticles from suspensions into a light-microscope prefocused central region of the pressure node plane. The ultrasound chamber had a single half-wavelength path length (lambda/2 = 0.25 mm) at its resonant frequency of 3.17 MHz. The full course of aggregate growth, in water or in 10 mM CaCl2, of 25 etam diameter latex particles was monitored by video microscopy. The particles formed two-dimensional aggregates in the nodal plane. Particles suspended in deionized water joined the aggregate both as single particles and as clusters. Incoming particles rolled over particles at the edge of the aggregate. The aggregate then adjusted its internal order to maintain a regular array of particles in apparent close contact. In contrast, particles in salt solutions interacted strongly on contact so that the aggregates grew as dendritic structures. The perimeter fractal dimensions of 1.15 and 1.59 in water and in salt, respectively, were typical of transport-limited and reaction-limited aggregation. When the ultrasonic exposure was terminated, the compact, low fractal dimension flocs disaggregated as they sedimented while the dendritic aggregates remained intact. It is shown that at surface separations of the order of 10 nm the acoustic particle interaction forces (dependent on the separation of centers of mass) are small compared to the van der Waals and electrostatic interaction forces (dependent on surface-surface separation) that normally control microparticle aggregation. It is concluded that the ultrasound did not significantly influence the net interactions that determine aggregate morphology and stability and that the ultrasonic trap technique holds promise for elucidating patterns of aggregate growth.