The vegetative Plasmodium of Plasmodiophora brassicae is contained within host cell cytoplasm. The multinucleate Plasmodium contains numerous large lipid droplets and is replete with mitochondria, dictyosomes, endoplasmic reticulum, and ribosomes. A 250 Å thick osmiophilic plasmodial envelope is characteristic of the vegetative Plasmodium. This envelope is composed of two closely appressed membranes each consisting of a pair of electron-dense layers. As sporogenesis is initiated, the outer membrane of the plasmodial envelope disintegrates, leaving the Plasmodium surrounded by the plasmodial membrane. Nucleoli disappear and numerous vacuoles and vesicles appear in the cytoplasm of the parasite. Vacuoles appear to be formed from invaginations of the plasmodial membrane. These vacuoles become aligned in planes of cleavage around each nucleus, forming the boundaries of each future sporangium. As these vacuoles coalesce, the nucleus of each young resting sporangium becomes surrounded with cytoplasm and is enveloped by the vacuolar membranes. Spines form on the sporangial membrane as aggregates of residual vacuolar material. The sporangial wall is then deposited between the sporangial membrane and the spines. Intact host nuclei, mitochondria, and plastids can be found dispersed among the mature resting sporangia. The host plasmalemma is the only membrane surrounding the mass of resting sporangia. The uninucleate resting sporangium is rich in lipid and contains those organelles found in the vegetative Plasmodium.