Invasion of Aeromonas hydrophila strain Ah J-1 isolated from diseased fish to cultured HEp-2cells monolayer was evaluated by the recovery of gentamincin-resistant (Gmr) bacteria from Triton X-100 cell lysates. The invasive efficiency ( IE) could reach to 0.2 % at 37℃. In addition, potential signal transduction pathways that precede bacterial internalization were studied by using signal transduction inhibitors. Bacterial internalization of Ah J-1 involved microfilaments and protein tyrosine kinase since cytochalasin D (an inhibitor of microfilament polymerization) and genistein (an inhibitor of protein tyrosine kinase) prevented internalization. Staurosporine (a protein kinase C inhibitor) and Sodium orthovanadate (a protein tyrosine phosphatase inhibitor) accelerated internalization of Ah J-1 into HEp-2 cells.