ENJ.P. Graffenauer, a doctor and naturalist, crossed Prussia and North-East Poland with the Grande Armee. He published his impressions as letters. The article presents and analyses his writings about Poland. He shares precious informations about costums, including traditional burial, greetings, use of languages, architecture and landscapes. Minorities such as Jews and Tartars are also described. He analyses the agriculture and culture of the "manne de Pologne", as well as the use of specific plants. Nature, through descriptions of forests, fauna (European bison, elks, bear, wolves), and geology, forms an important part of his letters. He describes in detail many diseases and presents his opinion on the causes of the "plica polonica". Finally, he gives several infomations about war and its consequences. His description is a precious testimony, little-known and rarely used by historians.