In this study, we present a method for fine focusing of microparticles using a vibrating air bubble. Fluorescent polystyrene particles of 2.01 and 6.60 μm are shown to be focused along the edge of a 50-μm-wide channel. The mechanism is determined to be shear stresses caused by the boundary layer on the vibrating gas/liquid meniscus. Experiments show that the focussing region width reduces with higher excitation amplitude, focusing is not as sensitive towards the flow rate as the excitation frequency, and there is an optimal bubble radius. Finally, we show that red blood cells and 6.60 μm particles can be focused into what is effectively a single line.