Frans Verdoorn (1906-1984), the renowned bryologist and publisher of the Chronica botanica series, was Professor of Biohistory at Utrecht University from 1957 until 1976. In this article his career and his 'biohistorical ideology' are reviewed. Biohistory was defined by Verdoorn as "the historical relationships of plants, animals and man in science, early medicine and culture." Verdoorn devoted most of his time and energy to elucidating and refining his ideology and his classification of the biohistorical subject matter. Much attention was also given to building up the Biohistorical Institute's library and archival collections. It is argued that Verdoorn's classificatory and bibliographical preoccupations prevented him from providing a clear cognitive identity for his field: he failed to make clear on what grounds biohistory deserved that status of an independent discipline, with its own problems and methods. This point is illustrated by a comparison of Verdoorn with two figures whom he regarded as kindred spirits, Carolus Linnaeus and George Sarton. After Verdoorn's retirement, the biohistorical ideology was abandoned and replaced by a programme in the history of biology.