Piezoelectric fibers with internal electrodes were fabricated by the co-extrusion process. The initial feedrods, which were composed of an outer piezoelectric PZN-PZT layer, a thin conducting PZN-PZT/Ag layer inside, and fugitive carbon black at the center, were co-extruded through a reduction die (1 mm) to form a continuous fiber. After thermal treatment and sintering, the PZN-PZT/Ag layer became the inner electrode, while the carbon black at the center was removed by oxidation to form an empty space. Three different types of fibers were produced: (i) solid fiber filled with an inner electrode, (ii) hollow fiber clad with a uniform inner electrode, and (iii) hollow fiber clad with a partial inner electrode. The piezoelectric properties of the fibers were evaluated in terms of their longitudinal strain (s(31)) or transverse displacement. When the dimensions of the fiber were 840 mu m (outer diameter) x 420 mu m (inner diameter) x 40 mm (length), the longitudinal strains of the solid fiber with the inner electrode and hollow fiber clad with the uniform inner electrode were 5.25 x 10(-5) and 8.5 x 10(-5) m/m, respectively, under an applied voltage of 100 V (0.48 kV/mm) at a frequency of 100 Hz. For the hollow fiber clad with a partial inner electrode with the same dimensions, the transverse displacement was 80 mu m under the same applied electric field.