Host defense peptides (HDPs) are a large group of small positively charged peptides that play an important role in innate immunity. Their role is more critical at early ages when other components of the immune system have not fully developed. There are three classes of avian HDPs: avian beta defensins (AvBDs), cathelicidins (Cath) and liver-expressed antimicrobial peptide 2 (LEAP-2). The objective was to compare expression of HDPs in male turkey poults at day of hatch (D0), D7, D14, D21 and D28 from the thymus, spleen, bursa, duodenum, jejunum and ileum. The expression of AvBD1, AvBD2, AvBD8, AvBD9, AvBD10, AvBD13, Cath2, Cath3 and LEAP-2 was measured using qPCR (n=6 birds/tissue/age). Data were analyzed by one-way ANOVA and Tukey's test, and significance considered at P ≤ 0.05. AvBDs and Caths exhibited greater expression in immune organs than intestinal tissues, with the greatest expression of AvBDs observed in the spleen. The intestinal tissues showed very low expression of AvBDs except for AvBD10 at D0. Similar to AvBDs, Caths expression in the immune organs was greater than the intestinal tissues with the spleen having the greatest expression among immune organs. Conversely, LEAP-2 showed greater expression in the intestinal tissues than in the immune tissues, which showed very low LEAP-2 expression unlike other HDPs. Understanding the differential expression of HDPs could reveal the innate immune status of poults, and may subsequently allow improvement of their health through appropriate mitigation strategies. / Master of Science