Extended micron-scale structures were produced in thermoplastic melts from initially large clusters of conducting carbon black particles transported by three-dimensional chaotic mixing. The structures formed networks that were captured by solidification and rendered the composite materials electrically conducting. A systematic study was carried out to assess the influence of key parameters and relate the electrical properties to the microstructures. Micrographs showed complex structures exhibiting patterns characteristic of chaos. Electrical measurements indicated that conductivity was achieved at carbon black concentrations significantly lower than achievable by common mixing methods, and lower than reported recently for two-dimensional chaotic mixing.