Background: We have analyzed by electron microscopy, immunohistochemistry, and flow cytometry the development of chicken caecal tonsil, the largest lymphoid organ of avian gut-associated lymphoid tissue (GALT). Methods: White Leghorn chickens of different ages obtained from a local supplier were routinely processed by transmission electron microscopy. For both immunohistochemistry and flow cytometry, we tested a battery of specific monoclonal antibodies (mAbs) to chicken cell markers on caecal cryosections or cell suspensions, respectively. Results: A rudimentary caecal tonsil occurs at the end of incubation. The organ grows just after birth, reaching the adult condition 4 days later. Firstly (4 days to 2 weeks), it contains predominantly T lymphocytes, principally TcR␣ ؉ and CD4 ؉ cells, which occupy largely the named caecal diffuse lymphoid tissue. In adult tonsils (6-week-old chickens) however, B lymphocytes, mainly expressing either IgM or IgA, predominate. They occur in both the subepithelial zone and the germinal centers, in which there are also a few T cells. After 2 weeks the CD8 ؉ lymphocytes gradually become more numerous than CD4 ؉ cells. In the tonsillar epithelium CD8 ؉ TcR␥␦ ؉ T cells, CD8 ؉ TcR␥␦ ؊ ␣ ؊ , presumably NK cells, and a few B lymphocytes are the main cell subpopulations. Conclusions: Chicken caecum grows fast after hatching. The diffuse lymphoid tissue largely contains TcR␣ CD4 ؉ or CD8 ؉ cells. CD8 ؉ cells of caecal epithelium represent ␥␦ T cells or NK cells. B lymphocytes which occur in the subepithelial zone, germinal centers, and, in few numbers, the caecal epithelium predominantly express either IgM or IgA. Anat. Rec.