RNA-arbitrarily primed PCR techniques have been applied for the first time to identify differential gene expression in black band disease (BBD), a virulent coral infection that affects reef ecosystems worldwide. The gene activity for the BBD mat on infected surfaces of the brain coral Diploria strigosa was compared with that for portions of the BBD mat that were removed from the coral and suspended nearby in the seawater column. The results obtained indicate that three genes (DD 95-2, DD 95-4, and DD 99-9) were up-regulated in the BBD bacterial mat on the coral surface compared to the transcript base levels observed in the BBD mat suspended in seawater. Clone DD 95-4 has homology with known amino acid ABC transporter systems in bacteria, while clone DD 99-9 exhibits homology with chlorophyll A apoprotein A1 in cyanobacteria. This protein is essential in the final conformation of photosystem I P700. DD 95-2, the only gene that was fully repressed in the BBD mat samples suspended in seawater, exhibited homology with the AraC-type DNA binding domain-containing proteins. These transcriptional activators coordinate the expression of genes essential for virulence in many species of gram-negative bacteria.