The grouper iridovirus (GIV) belongs to the family Iridoviridae, whose genome contains an antiapoptotic B-cell ymphoma (Bcl)-2-like gene. This study was carried-out to understand whether GIV blocks apoptosis in its host. UV- rradiated grouper kidney (GK) cells underwent apoptosis. However, a DNA fragmentation assay of UV-exposed GK cells after GIV infection revealed an inhibition of apoptosis. The UV- or heat-inactivated GIV failed to inhibit apoptosis, mplying that a gene or protein of the viral particle might contribute to an apoptosis inhibitory function. The DNA ladder ssay for GIV-infected GK cells after UV irradiation confirmed that apoptosis inhibition was an early process which ccurred as early as 5 min post-infection. A GIV-Bcl sequence comparison showed distant sequence similarities to that of uman and four viruses; however, all possessed the putative Bcl-2 homology (BH) domains of BH1, BH2, BH3, and BH4, s well as a transmembrane domain. Northern blot hybridization showed that GIV-Bcl transcription began at 2 h post- nfection, and the mRNA level significantly increased in the presence of cycloheximide or aphidicolin, indicating that this GIV-Bcl is an immediate-early gene. This was consistent with the Western blot results, which also revealed that the virion carries the Bcl protein. We observed the localization of GIV-Bcl on the mitochondrial membrane and other defined ntracellular areas. By immunostaining, it was proven that GIV-Bcl-expressing cells effectively inhibited apoptosis. Taken together, these results demonstrate that GIV inhibits the promotion of apoptosis by GK cells, which is mediated by the mmediate early expressed viral Bcl gene.