The article presents a study and publication of the correspondence of the poet Ivan Alekseevich Kostin (1931–2015) from Petrozavodsk with the archaeographer Vladimir Ivanovich Malyshev (1910–1976), who held a Doctor of Sciences degree in Philology, and the Old Believer writer and educator Ivan Nikiforovich Zavoloko (1897–1984). The correspondence includes letters and greeting cards (30 in total) from the 1970s to the early 1980s. They are currently stored in the Manuscript Division of the Institute of Russian Literature (Pushkinskij Dom) of the Russian Academy of Sciences in St. Petersburg, the Archive of the Grebenshchikov Old Believer Congregation in Riga, the National Museum of the Republic of Karelia in Petrozavodsk, and the National Archive of the Republic of Karelia, also in Petrozavodsk. Kostin’s letters to Malyshev reveal how the Petrozavodsk poet aided Malyshev in collecting manuscripts for the Ancient Manuscripts Repository (Drevlekhranilishe) in the Institute of Russian Literature (Pushkinskij Dom). The correspondence between Kostin and Zavoloko concerns the history and culture of the Old Believers, the Vygoleksinsky monastery, and the Zaonezhye, and issues pertaining to literary activity and academic studies. The letters make a valuable addition to Kostin’s memoirs about Malyshev and Zavoloko. The article also covers the history of Kostin’s poem dedicated to Archpriest Avvakum. The letters, published in the appendix to the article, are accompanied by comments.