Ultrastructural details of sporangial plasmodia and zoosporangia of Spongospora subterranea f. sp. nasturtii infecting the roots of watercress were observed using transmission electron microscopy. The plasmodial envelope, the interface between host and parasite, was found to be convoluted, having a large surface area. Pseudo-food vacuoles, formed by convolutions of the plasmodial envelope, contained portions of host cytoplasm. True-food vacuoles containing the remains of host organelles were seen within plasmodia. Similar remains also were seen in the central vacuole. The flagellar apparatus including the kinetosome, with microtubular rootlets, the transition zone, the flagellum and the tapering end-piece were examined and described. These observations are discussed in terms of the nutrition of S. subterranea f. sp. nasturtii, its original classification within the fungi (latterly Plasmodiophoromycetes) and affinities with the protists.