The work examines the circumstances of the transfer of the largest collection of butterflies, collected by Grand Duke Nikolai Romanov for 26 years, to the Zoological Museum of the Imperial Academy of Sciences in St. Petersburg in 1899–1900. The gift was made on the condition that the collection would retain the name of the donor; that, because of its considerable size, there will be a special custodian; that it would remain untouchable and without resupply, except for the species that may come from the Russian Empire; that it will be available for the work of scientists and professionals interested in the field. Two requirements were added a little bit later. The collection should have been kept in the same cabinets as it was at the Grand Duke’s palace. Otto Hertz was to be left the custodian with the position of senior zoologist. The main sources of the article are the minutes of the meetings of the Physics and Mathematics Department of the Imperial Academy of Sciences, museum reports, books of receipts from the Scientific Archives of the Zoological Institute of the Russian Academy of Sciences, materials of the St. Petersburg branch of the Archive of RAS and the Russian State Historical Archives.