In this study we investigated the early immune response of the Caribbean reef-building coral Pseudodiploria strigosa (symmetrical brain coral) by sequencing the transcriptome of healthy colonies challenged with live bacteriausing the Illumina paltform. De novo assemblingof sequence reads resulted in 253,028 contigs, from which only 45% were annotated. A total of 2,782 transcripts encoding conserved immune-related proteins were identified by searching the annotated transcriptome with a list of 109 immune-related keyword terms. In addition, an in silico differential expression analysis comparing the transcriptome of bacterially-challenged samples with that of an untreated control, showed that 728 immune-related transcritps were up-regulated after the immune challenge. The identified immune-type molecules were classified into the three functional modules of immunity: a) the immune recognition module, containing a wide diversity of membrane-bound and secreted putative pattern recognition receptors, including leucine-rich repeat-containing proteins, immunoglobulin superfamily receptors, representatives of various lectin families, and scaveneger receptors, b) the intracellular signalingmodule, containingcomponents from the Toll-like receptor, Transforming Growth Factor (TGF), MAPK, Notch, and apoptosis signal transduction pathways, and 3)the effector module including the Complement system components C3 and Factor B, a variety of proteases and protease inhibitors, and themalanization-inducing enzymepro-phenoloxidase. Thus, D. strigosa displays a highly variable and diverse immune repertoire that is likely to contributeto its known resilience to the threats that are currently decimating reef-building corals.