The ability to discriminate between beneficial and pathogenic microbes is critical to animal fitness, yet whether copepods are able to actively select for or against particular microbial colonizers is not well understood. In this study, the global transcriptomic response of the estuarine copepod Eurytemora affinis to colonization by a range of Vibrio species was characterized via Illumina RNA-Seq to identify the mechanisms by which copepod hosts may respond to and discriminate between colonizing Vibrio species.