Around the turn of the twentieth century, microbiologists in Western Europe and North America began to organize centralized collections of microbial cultures. Collectors published lists of the strains they cultured, offering to send duplicates to colleagues near and far. This essay explores the history of microbial culture collections through two cases: Johanna Westerdijk’s collection of phytopathogenic fungi in the Netherlands and Ernst Georg Pringsheim’s collection of single-celled algae at the German University in Prague. Historians of science have tended to look (...) at twentieth-century biological specimen collections as either repositories of communal research materials or storehouses of economically important biological variation. An examination of Westerdijk’s and Pringsheim’s collections illustrates how collectors, researchers, and patrons ascribed different kinds of value to collections featuring distinctive microbial life forms. This essay argues that characteristics of cultivated microorganisms, such as a fungus’s propensity to infect crops or an alga’s amenability to experimentation, shaped the trajectories of Westerdijk’s and Pringsheim’s collections as these collectors developed relationships with colleagues and patrons. Letters between Westerdijk and Pringsheim open a window onto divergences in their approaches to collecting cultures, while also shedding light on the aspirational internationality of the collections that resulted. (shrink)