Despite the growing interest in microfluidic systems, the study of two-phase flows in such systems thus far has been somewhat limited in scope. Examples of recent studies of two-phase flows in microchannels include flows containing liquid and gas (Tran, 1998; Stanley, 1997), the dispersion of laminar fluid streams (Galambos, 1998), and the flow of very dilute particle suspensions through micropumps (Jang et al., 1999). The present experiments identify 'shear-induced arching' as a new mechanism causing microtube blockage. This mechanism is most likely to occur when 0.33D < d p< 0.46D, where d p is the particle diameter and D is the microtube diameter, and was observed for flows of particle-laden fluids with concentration, φ, as low as 0.5%. Following a simple geometrical analysis, for 0.33D < d p<0.46D, it can be shown that once the particles are in the arching configuration, lateral forces induced by the shear on the arch can hold the particles in place and stabilize the arch. Experiments were performed over a range of particle-to-tube diameter ratios. As predicted, if enough of the particles in any given experiment had a diameter of 0.33D-0.46D, blockages occurred in the microtube.