The objective of this study was to determine microtubule assembly and chromatin configuration in porcine oocytes during the first cell cycle following round spermatid injection into matured porcine oocytes in the presence or absence of electrical stimulation. The oocytes with two large pronuclei and two polar bodies were classified as normal fertilization at 6 to 8 h following injection. The incidence of normal fertilization following round spermatid injection with electrical stimulation was significantly higher (21/45, 47%) than that following injection alone (6/39, 15%). Although a small microtubular aster was organized near the decondensed spermatid chromatin in some oocytes (2/6, 33%, spermatid injection alone; 9/21, 29%, spermatid injection and electrical stimulation), it did not enlarge nor fill the cytoplasm. Instead, a dense network of microtubules in the cytoplasm was organized from cortex. At 12 to 15 h after injection, we classified the oocytes with closely apposed pronuclei as normal fertilization. The electrical stimulation following spermatid injection enhanced (P < 0.05) the incidence of normal fertilization (18/54, 33%) compared with spermatid injection alone (7/52, 13%). During pronuclear movement, the maternally derived microtubules filled the whole cytoplasm, which appeared to move male and female chromatin. Mitosis and two-cell division were observed at 20 to 24 h after spermatid injection with electrical stimulation (12/41, 29%). At mitotic metaphase, the microtubular spindle had focused astral poles, and chromosomes were aligned on the spindle equator. During mitosis, asters were assembled at each spindle pole, and they filled the cytoplasm. These results suggested that round spermatid nuclei of the pig can develop into a morphologically normal pronucleus in matured porcine oocytes and are competent to participate in syngamy with the ootid chromatin. In addition, functional microtubules for complete fertilization with spermatid were not associated with male-derived centrosome but were organized solely from maternal stores.