The detailed structure of the nucleus and its associated suborganelles in the rust fungi Puccinia graminis tritici, P. graminis avenue, P. recondita, and P. coronata is described. The non-mitotic nuclei in intercellular hyphae of all of the fungi examined were irregularly oval in shape and had prominent nucleoli, and except for P. recondita, heterochromatin was usually uniformly dispersed. In P. recondita, densely staining patches occurred throughout the nucleus, and this was a distinguishing feature of this species. The nuclei in monokaryotic axenic cultures of P. graminis tritici and P. coronata were larger than those in their respective dikaryotic parasitic hyphae or in a dikaryotic axenic culture of P. graminis tritici.The nucleoli varied in size and composition, depending on the physiologic condition or type of cell. In senescing cells the nucleoli occupied about 10% of the nuclear volume, while in young active cells the respective volume occupied was up to 60%. In haustoria the nucleoli were smaller in size and were composed mainly of fibrillar material. In active intercellular hyphae of all of the fungi examined the nucleoli consisted of about equal granular and fibrillar regions. There was a lighter-staining central region and similar light spaces in the fibrillar zones. These light areas were similar to the nucleoplasm in appearance and were interpreted as lacunae possibly continuous with the nucleoplasm. The rust fungal nucleoli and those of some higher plants were compared.A bipolar spindle pole body (SPB) was a regular feature of non-mitotic nuclei. The SPB consisted of two disc-like structures located some distance apart on a layer of amorphous substance. The SPB was located outside the nucleus in a depression of the nuclear envelope, usually toward one side of the nucleus. Subtending the SPB in the nucleus and joined to the SPB via a large pore in the nuclear envelope was a moderately dense region which consisted of an amorphous layer from which loose threads radiated into the nucleus. Occasionally a thread connected this region to the nucleolus.In several cells the nuclei were deformed with concomitant extrusion of the nucleoli. Nucleolar extrusion was seen in all material examined, and the process is described. Nuclear deformation and nucleolar extrusion were considered to be indicators of premitotic nuclei.