Matthias Alexander Castrén (1813–1852) was a Finnish linguist and ethnographer. When he died at the young age of 38, he left behind a huge corpus of field data, collected by himself during prolonged expeditions to Karelia, Lapland, Arctic Russia and Siberia from 1838 to 1849. In spite of his active production of specialized articles, reviews and travel reports during his lifetime, a major part of his scholarly heritage remained unpublished when he died. In accordance with the original agreement with the Russian Imperial Academy of Sciences, Castrén’s manuscripts were placed in the library of the Imperial Alexander University of Helsinki. These materials, kept today in the National Library of Finland, Helsinki, have been bound into 33 mainly folio-sized volumes, which, over the years, have been preliminarily catalogued and microfilmed. The dataset contains facsimile files of the manuscripts related to Tundra and Forest Nenets linguistic, ethnographic and folklore collections.