The European bison Bison bonasus went through a severe bottleneck and became extinct in the wild 90 years ago. The lowland subspecies B. b. bonasus is the only one of three original subspecies that exists today. The entire species derives from only 12 founders, including a bull of the Caucasian subspecies B. b. caucasicus. Due to its presence among founders, there are two geographically separated genetic lines of European bison: the pure lowland (Bialowieza) line and the hybrid lowland-Caucasian line. The lowland line of the European bison originates from only seven founders with an extremely varying genetic contribution. Approximately 80% of the genes in contemporary populations come from just two founders. A variety of genetic markers (mtDNA, microsatellites, single nucleotide polymorphism microchips) were applied to studies of the level of depletion of genetic variability in European bison. The lowland line of the European bison, the most extensively studied, shows very low levels of genetic variation, and has just half the microsatellite heterozygosity of the closely related American bison Bison bison. The effective population size (N-e) for the highly genetically homogenous lowland line in the Polish part of the Bialowieza Forest is estimated to be 23.5, far less than the census population size of 450. The average inbreeding level in lowland bison is almost 50 although no signs of inbreeding depression have been observed. In contrast, inbreeding effects have been noticed in the lowland-Caucasian line, which has a much lower average inbreeding level (28. In spite of the apparently high fitness of the lowland bison, the lack of genetic variation and high level of inbreeding may present substantial threats in the future, especially in the context of potential epizootics.