Technical Abstract: Chemokines play important roles in controlling leukocyte trafficking under normal and inflammatory conditions. Sixteen CXC chemokines have been identified in the human and mouse genomes, while considerably fewer teleost fish CXC chemokines have been reported. Here, we describe a novel clade of trout (Onchorynchus mykiss) CXC chemokines, designated Onmy CXCd, and we identify a novel gene, CXCd1, and a putative duplicate, CXCd2. The trout CXCd proteins contain 112 amino acids and the CXCd1 gene is comprised of four exons and three introns. Constitutive CXCd mRNA expression was detected in skin, gill, visceral fat, and posterior kidney tissues, while low transcript levels were present in the anterior kidney and spleen. Spleen CXCd transcript abundance increased 1 day after bath vaccination (fourfold) and subsided to basal levels by 7 days postvaccination. Challenge with viable Yersinia ruckeri induced expression of trout CXCd RNA up to ninefold in the spleen. The number of viable Y. ruckeri were significantly correlated with CXCd gene transcript abundance (P = 0.0051, Spearman correlation 0.497, n = 30 fish), and fish with the highest bacterial loads had the highest CXCd expression. In contrast, pro-inflammatory cytokine IL-1-beta2 mRNA levels were elevated in fish infected with low numbers of Y. ruckeri, while diminishing in heavily infected fish. CXCd mRNA expression was not increased in rainbow trout infected with infectious hematopoietic necrosis virus, suggesting that up-regulation may be pathogen-specific. Taken together, these results indicate that CXCd transcript elevation follows the pro-inflammatory cytokine response to Y. ruckeri and may be a relevant immunological marker of exposure.