Nuclear structure and division in the mycelium and basidiospores of Ceratobasidium praticolum (Pellicularia praticola), studied in living and in differently stained preparations, are described and illustrated with photographs. The cells of the mycelium are multinucleate. Nuclear migration occurs from one cell to another through the septal pores. The nuclei consist of a Feulgen-negative nucleolus surrounded by granular Feulgen-positive chromatin. In a nucleus preparing to divide, the nucleolus becomes progressively smaller and finally disappears. The chromatin then elongates, becoming bar-like, and later constricts at the mid-region. New nucleoli appear in the two attached portions of the constricted chromatin. At the end of the division the two portions pull apart to form two sister nuclei. Classic mitotic division as it occurs in higher organisms was not seen. Chromosomal filaments were not discerned in the nuclei of the basidiospores or the mycelium. Chromosomal filaments in reticulate arrangement were observed in the nuclei of the hymenial cells and in the cells of the short, side branches on which the hymenium develops.