Fungus-growing ants (Formicidae: Attini) comprise a diverse and ecologically important group in Neotropical habitats. Compared with leaf-cutters, however, relatively little is known about the biology of less conspicuous attine species. Here, we compare nest size and structure, colony size and demographic composition, and worker size and polymorphism among the genera Cyphomyrmex, Mycetarotes, Mycocepurus, Myrmicocrypta, Sericomyrmex, and Trachymyrmex. In total, 25 ant colonies (one species per genus) were investigated at one site in the Brazilian savanna. Results indicate a consistent variation in nest size and structural complexity (architecture), colony and worker size, and a tendency to polymorphism among the studied genera. In addition, nest mound volume was found to be a good predictor of both worker number and volume of the fungus garden. Based on morphometric analyses, Sericomyrmex and Trachymyrmex together formed a distinct group from the other genera. The transition from the ancestral agricultural system toward the derived leaf-cutting habit also is followed by remarkable changes in nest size and architecture, colony size, and worker size and polymorphism. Our results support other recent studies that consider Sericomyrmex and Trachymyrmex as possessing transitional habits, distinct both from species that cultivate fungus by using mostly nonplant items (insect feces and corpses) as well as from typical leaf-cutters Atta and Acromyrmex. This is the first study to detect correlations of nest traits with worker number and size of fungus garden in the less conspicuous attines. Results highlight the importance of combining data on natural history and morphometry to understand the evolutionary history of fungus-growing ants.