The delayed-early DNA polymerase promoter of Chilo iridescent virus (CIV), officially known as Invertebrate iridescent virus, was fine mapped by constructing a series of increasing deletions and by introducing point mutations. The effects of these mutations were examined in a luciferase reporter gene system using Bombyx mori cells transfected with promoter constructs and infected with CIV. When the size of the upstream element was reduced from position -19 to -15, relative to the transcriptional start site, the luciferase activity was reduced to almost zero. Point mutations showed that each of the 5 nt (AAAAT) located between -19 and -15 were equally essential for promoter activity. Mutations at individual bases around the transcription initiation site showed that the promoter extended until position -2 upstream of the transcription start site. South-Western analysis showed that a protein of approximately 100 kDa interacted with the -19 nt promoter fragment in CIV-infected cells. This binding did not occur with a point mutant that lacked promoter activity. The AAAAT motif was also found in the DNA polymerase promoter region of other iridoviruses and in other putative CIV delayed-early genes.