Colonies of the compound ascidian Botryllus schiosseri undergo regular generation changes, during which adult zooids are progressively resorbed and replaced by growing buds. The generation change, or take-over, is characterized by massive cell death by apoptosis, as indicated by nuclear condensation, activation of caspases, overexpression of molecules recognized by antibodies against mammalian Bax, Fas, and FasL, changes in the expression of surface molecules by senescent cells of zooid tissues, and recruitment of circulating phagocytes in zooid tissues which ensure the complete clearing of dying cells. The entire process lasts 24-36 h at 20 degrees C and has been subdivided, on the basis of the degree of contraction of old zooids, into four substages. In the present work, we carried out a detailed morphological analysis of the events occurring in zooid tissues during the take-over substages. Results Indicate that traces of apoptosis can be found in the epidermis, peribranchial epithelium, and heart in the late substage but are easily found in the branchial basket 2-4 h after the beginning of the generation change, thus confirming the antero-posterior progression of cell death, at least in the alimentary system.