The ultrastructure of the pigeon bursa of Fabricius during involution is described. According to the AA, the involutive process may be divided into 3 stages: an early involutive stage, a late involutive stage and a residual stage. The involutive process starts from epithelium later affecting the follicular medulla and all other structures of the organ. A mucoid degeneration can be observed, followed by an impressive fatty degeneration.