Many studies have been done on the virulence factors of Vibrio parahaemolyticus, which causes acute gastroenteritis. Invasion by this bacterium of culture cells in vitro, however, has not been clearly demonstrated. To assess the invasive ability of V. parahaemolyticus, quantitative studies using antibiotic survival assays were done. Of 21 isolates examined, 4 could invade Caco-2 cells, a human colon carcinoma-derived cell line. Invasion of an isolate, AQ4023, was inhibited by cytochalasin D, nocodazole, and genistein. This indicates that active processes in cells, such as signal transduction by tyrosine protein kinase, may be involved in the internalization of this bacterium by Caco-2 cells and that actin filaments and cytoskeletal structure may have important roles in this process. These results suggested that the disease caused by some isolates of V. parahaemolyticus is attributable not only to toxin production but also to invasion into intestinal epithelium.