|Beschreibung:||The freshwater cnidarian polyp hydra is the simplest organism hitherto known to possess components of the apoptotic machinery. However, little is known about the functions of apoptosis in hydra. Here I present evidence for the role of apoptosis in hydra spermatogenesis and histocompatibility. Cells undergoing programmed cell death were detected by vital acridine orange staining and TUNEL assay. Sperm precursors were shown to undergo apoptosis mostly at early stages of spermatogenesis. Role of apoptosis as a mechanism to adjust the number of sperm precursors at different stages of spermatogenesis is discussed. Surprisingly, epithelial cells, previously thought to function merely as physical support of the testes, were found to play an active role during spermatogenesis as phagocytes. Similar to Sertoli cells in mammals they may provide signals to differentiating sperm. Apoptosis was also found to be induced at the graft border in hydra interspecies grafts. Though self / non-self recognition was proposed previously to play a crucial role in such heterografts, no evidence for existence of allorecognition system was found. Hydra seems to have lost this system secondarily during evolution. Ultrastructural studies of the graft site revealed morphologically disturbed mesoglea separating H. vulgaris ectoderm and H. oligactis endoderm. Apoptosis is proposed to result due to incompatibility between tissues of these two species. Role of incompatibility mechanisms such as anoikis or inability to properly recognize survival signals from a different species is discussed.