The traditional view of London's Royal Society as a closed circle has been subject to revision in the past decades. Historians have shown the considerable extent to which the Fellows of the Society drew on a broad range of men of practice for their respective skill sets. This article offers an in-depth analysis of the contributions of fishermen and fishmongers to the creation of natural knowledge. It centres on the Historia piscium (Oxford, 1686), written by Francis Willughby and John Ray, and its surrounding sources. This natural history of fishes aspired to give a concise and precise overview of species, and to uncover the divine order in which they were created. While men of practice contributed to this project in multiple ways, their first-hand observations carried particular weight. Through their cumulative experience of working with fish they saw a great number of living species, rather than the dried exemplars that naturalists would usually consult in cabinets of curiosities, or the indirect evidence that images might present. This article examines what kind of exchanges took place between fishermen and fishmongers on the one hand and Fellows on the other, and where, how and why these were incorporated into the fish book. In so doing, it also aims to qualify the value attached to direct (natural historical) observation in the socio-cultural context of late seventeenth-century England.