The crucian carp (Carassius carassius) has an exceptional ability to tolerate anoxia, being able to survive without oxygen for several months at low temperatures. In response to low oxygen levels the crucian carp reduces its metabolic rate, and up-regulates glycolysis to produce enough ATP to fuel cellular ATP demand. These adaptations solve the main problem encountered during anoxia, which is to supply cells with enough ATP. The brain has a very high rate of ATP use, and it is therefore especially vulnerable during anoxic conditions. In the brain of mammals, even brief periods of oxygen deprivation can induce apoptotic cell death. It is still not known if the crucian carp brain suffers brain damage after anoxic exposure. The aim of this study was therefore to examine if anoxia, and/or subsequent reoxygenation, affect the incidence of apoptotic cell death in the brain of crucian carp.The terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) method was applied to stain for and quantify apoptotic cells in the crucian carp telencephalon. The amount of apoptotic cells did not increase significantly after 7 days of anoxia (at 9 °C). However, when the anoxic fish were given 1 day of reoxygenation at normal oxygen levels, a 170 % increase in the number of apoptotic cells was detected. The elevated apoptosis after reoxygenation resembles the effect of reperfusion after cerebral ischemia in mammals, where reperfusion accelerates the rate of cell death.One possibility is that anoxia initiates apoptotic pathways in the brain without leading to actual cell death until oxygen is restored. Another possibility is that anoxia in itself does not induce apoptosis, but that the following reoxygenation causes increased apoptosis. Regardless, anoxia followed by reoxygenation does cause some damage in the form of increased levels of apoptosis in the crucian carp telencephalon. This points at an hitherto unrecognized aspect of anoxia tolerance in crucian carp: the need to possess effective mechanisms to repair a damaged brain after anoxia / reoxygenation events.