The monolayer of GP cell cultures were infected with the grouper iridovirus isolate A3/12/98 as described in the above. After appearance ofadvanced CPE from 36 to 48 h of post-infection, the virus-infected GP cells were harvested and pelleted by centrifugation at 200×g for 5 min. The cell pellets were first fixed with 2.5% glutaraldehyde (SPI, EM grade) in 0.1 M cacodylate buffer, pH7.2 for 2–3 h, the cells were then washed with cacodylate buffer, and finally postfixed in 1% osmium tetroxide for 1 h. The cell pellet was mixed carefully with 2% molten agrose solution at 45 °C and immediately cooled on ice. One-mm3 cubes of the agrose-cell mixture were then dehydrated in a graded series of ethanol and embedded in resin (EMS, SPUR’S Kit). Ultrathin sections were cut with ultramicrotome (Reichert-Jung), and then double-stained with saturated aqueous uranyl acetate and lead citrate.