In 1520 the Bridgettine priest Petrus Magni (1460-1534), wrote a manual on agriculture. The manuscript, in Late Old Swedish, is to a large extent taken from Columella's De re rustica with many additions. At the end of the manual there is a brief chapter on making and keeping ponds for crucian carp (Carassius carassius) and tench (Tinca tinca). Aquaculture, with keeping and breeding fish in artificial ponds, was probably an innovation that became established in secular and monastic environments in Sweden in the fifteenth century. The text is to some extent based on Petrus's own experience and provides rare knowledge of pond-breeding of cyprinids in Scandinavia in late Medieval times. Petrus's account is the oldest known manual on fish-breeding in northern Europe. This brief manual is compared with manuals on fish culture by the Bohemian Bishop Janus Dubravius (1486-1553) and Polish nobleman Olbrycht Strumienski (d. 1609) published in 1547 and 1573 respectively.