Porphyromonas gingivalis, one of the important periodontal pathogens, exhibits many virulence properties. Among these, the adhesion to and invasion into host tissues are crucial for the initiation and progression of periodontal diseases. While evidence indicating the ability of this organism to adhere to and invade into epithelial cells as well as endothelial cells has accumulated, that involving the gingival fibroblasts is very limited. Therefore, this study aimed to determine the ability of P. gingivalis to invade primary cultures of human gingival fibroblasts using the antibiotic protection assay. In addition, interactions between P. gingivalis and the gingival fibroblasts were investigated using electron microscopy. The results demonstrated that P. gingivalis 381 could invade human gingival fibroblasts with an invasion efficiency of 0.17%. Using the scanning electron microscopic study, numerous filopodia were seen on the surfaces of gingival fibroblasts after P. gingivalis adhesion. The transmission electron microscopy revealed the presence of an intracellular bacterium. After 90 min incubation, the bacterium was found in the cytoplasm of the gingival fibroblasts, without membrane surrounding. Some fibroblasts contained a number of vacuoles and dilated rough endoplasmic reticulum even when bacteria were not found intracellularly. Thus, the invasion of this organism into the gingival fibroblasts may play a direct role in the destruction of the periodontal tissues and may also relate to the difficulties of eradicating the bacteria from periodontitis lesions.