 In 1955, Iltis relocated to the Botany Department of the University of Wisconsin–Madison, where in addition to directing the herbarium he regularly taught plant geography, taxonomy, and grass systematics.He arranged to purchase much of the Catholic University of America herbarium when it was deaccessioned.By the time of his retirement in 1993, he had directed 37 candidates pursuing graduate degrees, and he and his students had collected thousands of specimens throughout the Upper Midwest to document distributions of plant species, leading to the publication of the "Atlas of the Wisconsin Prairie and Savanna Flora" (2000) coauthored with Herbarium Curator Theodore Cochrane.