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Phenotypes of myeloid-lineage cells remain poorly understood in the rainbow trout, and were the focus of this study, including effects of in vivo challenge to Flavobacterium psychrophilum (Fp), the cause of Bacterial Cold Water Disease (BCWD). A genetic line was used that is highly resistant to BCWD (R-line) as well as a susceptible control line (S-line). Using flow cytometry, we describe two Pax5-negative, myeloid-lineage populations: Population 1 consisted of small cells with high SSC and strong staining for Q4E, MPO, Pu1, EBF, and IL- 1β, which we named “neutrophil-like” cell. Population 2 had high Q4E, but weaker MPO, Pu1, EBF, and IL-1β staining. Five days after Fp-challenge, both genetic lines had a reduced abundance of neutrophil-like cells in anterior kidney, PBL, and spleen. Pop. 2 abundance was reduced in anterior kidney, and increased in spleen. S-line fish responded more strongly to Fp-challenge compared to R-line fish. Challenged fish with a higher abundance of neutrophil-like cells had significantly lower Fp-loads after challenge, suggesting that these cells aid in the resistance to BCWD.A line of Rainbow trout resistant to BCWD differed from a susceptible line in abundance of myeloid-lineage populations.After in vivo challenge with Flavobacterium psychrophilum, changes in abundance of neutrophil-like cells was more strongly seen in susceptible fish.Fp-challenged fish with higher abundance of neutrophil-like cells in their anterior kidney or PBLs had significantly lower Fp-loads.