The European bison (Bison bonasus) is the only living species of the bison genus in the Old World. It is the largest, wild, herbivorous mammal living in Europe. As a result of the efforts of biologists, the European bison has been saved as a species. In Poland, they were reintroduced into the natural conditions of the Bialowieza Forest. By 2011, this herd of free-living bison comprised more than 700 animals. The aim of the present work was to determine tissue levels of magnesium in free-living bison. Samples for the investigations were collected during the winter, from 20 European bison aged from five months to five years culled as part of the annual management programme. Segments of rib, muscle, liver, kidney, hoof and hair were collected. With regards to the gender and age of the animals studied, magnesium content in particular tissues was as follows: in the liver, magnesium content was significantly higher in the group of males; in rib, muscle and kidney there were no statistically significant differences between groups; in hair and hoof wall, a significantly higher magnesium content was found in the group of calves.