The scale and character of the Swedish historian Petrus Petrejus’ influence on Historia Belli Sveco-Moscovitici Decennalis by his countryman Johannes Widekindi have not been subject to systematical research; only a few of the loaned passages have been identified as such by Helge Almquist and the editors of the Russian translation of Widekindi. The general evaluation of Petrejus’ importance for Widekindi as formulated by the latter ones is nevertheless somewhat exaggerated. This article contains a list of the loans and a discussion of the problems that emerge when distinguishing the loans from Petrejus from the loans from the other sources (Stanisław Kobierzycki, Axel Oxenstierna, geographical and documentary sources). The analysis points out that Widekindi has not used the manuscript of Conrad Bussow’s chronicle. Finally, the mechanism of Widekindi’s work with Petrejus is specified. Despite being acquainted with the German version, he usually resorted to the Swedish one. Some details of Widekindi’s rendering of the information given by Petrejus demonstrate that when paraphrasing (and not quoting word for word, as on three occasions) Petrejus, the Latin text of Widekindi is the source text for his Swedish. So two steps of the work may be discerned: first Widekindi paraphrased Petrejus in his Latin draft, and then he added to the Swedish translation some fragments of the Swedish text by Petrejus.