Vaccination on young fingerlings requires understanding the development of adaptive immunity. We recently described that the nasal cavity of rainbow trout is lined with an extensive lymphoreticular epithelium that forms lymphoid aggregates. This structure, named organized nasopharynx-associated lymphoid tissue (O-NALT), was described in 30g fish and its composition consisted of 56% CD4+, 24% IgM+, 16% CD8α+, and 4% IgT+ lymphocytes. O-NALT has molecular markers of mammalian germinal centers and likely the home for the maturation of the adaptive immune response in response to mucosal vaccines. Thus, the goal of this study is to determine when O-NALT first appears during rainbow trout development and whether its cellular composition changes over time. Using routine histology as well as immunofluorescence (IF) staining with specific antibodies against CD4, CD8, IgM and IgT, we found that, histologically, O-NALT was first visible at 1490-degree days (DD) or 100 days post-hatch (mean weight 12g). IF staining showed that there is a developmental progression in lymphocyte composition in the O-NALT. Specifically, in 1490 DD young rainbow trout, CD8+ T cell numbers dominated O-NALT followed by IgM+ B cells. As fish developed, CD4+ T cells and IgM+ B cells become the dominant lymphocyte types in O-NALT. IgT+ B cell numbers did not change during development. These results indicate the early vaccination of rainbow trout may stimulate very different types of mucosal immunity depending on the age of the fish. Further studies are necessary to ascertain whether O-NALT of young fish also expresses molecular markers of mammalian germinal centers and whether it can support B cell selection in response to antigenic stimulation.