Uniform assemblies of all-identical colloidal particles, so-called photonic balls, are produced by injecting an aqueous suspension of polymer latex spheres into a surfactant-laden oil phase at an oil/water junction of capillary tubes or soft-microfluidic devices. The size of these assemblies can be controlled by varying the injection pressure or particle concentration. (C) 2003 Elsevier Science B.V. All rights reserved.