Postpenetration behavior of Fusarium circinatum in stems of pine species was investigated with light and transmission electron microscopy. Two-year-old stems of Pinus rigida and P. densiflora were wound-inoculated with the fungal conidial suspension and subjected to 25 degrees C for up to 30 days. It was common to observe the formation of wound periderm on each pine species, recovering wounded sites with newly formed tissues. The outermost thick layer of wound periderm was pink to red colored with the phloroglucinol-EtOH staining, indicating heavy deposition of lignin in wound periderm. The cork layers in the wound periderm of the two pine species consisted of cells that were mostly devoid of cellular contents in cytoplasm. The cork cells showed convoluted cell walls with different electron density (lamellations), which was seemingly more prevalent in P. densiflora than P. rigida. Hyphae of F. circinatum appeared normal with typical eucaryotic cytoplasm in P. rigida on ultrathin sections. Meanwhile, hyphae in P. densiflora were found to possess highly vacuolated cytoplasm, implying hyphal weakening and disintegration. Hyphal cytoplasm appeared to be a thin layer between the vacuole and the plasma membrane surrounded by cell wall. In addition, intrahyphal hyphae and concentric bodies were observed in hyphal cytoplasm. These results suggest that the architecture of wound periderm may be responsible for different responses of pine species to the invasion of F. circinatum.