It is shown that the magnitude of Rayleigh microstreaming convective drag on microparticles in water in a 3.2-MHz ultrasonic standing wave can be comparable to the lateral direct radiation force in the nodal plane (DRF1) and can significantly influence the micropanicle aggregation. The transducer of a single half-wavelength chamber was excited to give a single particle aggregate. The estimated sound pressure amplitude was 0.5 MPa. Particle image velocimetry (PTV) measurements gave the average microstreaming velocity in the nodal plane as 450 mum . s(-1), which is comparable to the 340- mum . s(-1) value calculated from Rayleigh's theory. Movement of 25-mum latex particles was primarily determined by DRF1 while that of smaller 1.0 mum, particles was determined by Rayleigh microstreaming. A 15-mum latex particle velocity map, simulated from microstreaming data, the measured velocity map of 25-mum particles, and the cube-dependent relationship between DPF1's on particles of different sizes, was in reasonable agreement with a measured velocity map. Further evidence for the importance of microstreaming came from the result that velocities for 1- and 25-mum particles were of similar magnitude, but were opposite in direction.