Expressed coding sequences were identified for the T cell receptors TCRα, TCRβ, TCRγ, TCRδ, TCRμ, and co-receptors CD3ε, CD4, and CD8α in the red-tailed phascogale (Phascogale calura). Expression of these genes was examined in the developing thymus and spleen of pouch young and juvenile individuals to detect any changes in expressionrelative to developmental stages. Transcripts of CD3ε, CD4, TCRδ and TCRμ were first detected in the thorax at 1 day postpartum (dpp), and transcripts of all target genes were detected in the thymus from 3 dpp, and the spleen from 17 dpp. The relative expression of all target genes changed significantly over the course of pouch life and was associated with histological milestones of both the thymus and the spleen. Peak gene expression was observed early in pouch life in the thymus, with a decrease in expression associated with timing of thymic involution, while expression peaked later in the spleen and was associated with the initial differentiation of the red and white pulp and a population of T cells forming the periarterial lymphatic sheath (PALS). Active expression of these target genes within the thymus and spleen indicates that the young marsupials are actively developing immunocompetence during pouch life.