Among the many topics of lively scientific work that Jean Emmanuel Gilibert (1741-1814) conducted in Grodno and Vilnius, an important place is occupied by his observations of wild mammals. Royal patronage and care from Antoni Tyzenhauz, Treasurer of the Grand Duchy of Lithuania and the governor of Grodno, allowed Gilibert to keep and observe wild fauna captured by royal services in royal forests, including Białowieża Primeval Forest. Such was an origin of a female bison kept by Gilibert in Grodno. Its description, published in Indagatores naturae in Lithuania (Vilnius 1781) for decades became the primary source of information about the behaviour, food preferences and the anatomy of European bison. European science has just begun to take interest in European bison, therefore Gilibert's account entered scientific circulation by way of French natural history encyclopaedias (mainly Georges Buffon's Histoire naturelle) and works by Georges Cuvier or Étienne Geoffroy Saint-Hilaire. Apart from the description of European bison, Gilibert left an entire series of observations of wild mammals inhabiting the forests of the Grand Duchy of Lithuania. His accounts of moose were important in building a knowledge base for this species. In the first half of the 18th century, moose was known mainly from fantastic descriptions in Renaissance works and from prescriptions devoted to using moose hoof as the epilepsy treatment. Gilibert's observations helped to overthrow such superstitions. Similarly, Gilibert's first-hand information verified the widespread legends concerning brown bear (e.g. the belief that white bears, belonging to other species than polar bears, occur in Lithuania) . List of species kept and thoroughly watched by the scholar is much longer and includes lynx, wolf (and hybrids of wolves and dogs), beaver, badger, fox, hedgehog, and even white mouse. Also his comments on the species of mammals then absent in Lithuania but known either from farming or from the fur trade (wolverine, bobak marmot or steppe polecat). Also in these cases, Gilibert's descriptions were often the first reliable information that entered the circulation in European science. His accounts were not free of errors and mistakes--but they resulted mainly from the pioneering role of his work. Some of his breeding experiments can arouse the reader's sincere smile today, such as an attempt to feed a beaver with fish or serve cooked beans to a lynx In the margins of his mammal observations, Gilibert described also the place of their occurrence, extensive forests of the Grand Duchy of Lithuania. Read from the contemporary perspective, his thoughts are surprisingly relevant. In his praise of "primeval nature, free from human actions and not disturbed by accident or by the impatience of human desires" he sounds very similar to today's eulogists of the primeval forest of Białowieża.