Ultra-thin sections of the tissues of Cruciferous plants infected by Albugo fungi were studied under an electron-microscope. Stem tissues of Brassica juncea consisted of cells showing hypertrophy and of hyperplastic tissues. Hypertrophied cells had a large central vacuole and a thin peripheral layer of cytoplasm contained mitochondria, amyloplasts and endoplasmic reticula. In the chloroplast of Wasabia japonica infected by Albugo wasabiae, grana lamellar structures, osmiophilic granules and developed starch grains were found. The fungi were observed in the inter-cellular spaces, middle lamella and cells of host tissues. The haustoria within the host cells were always surrounded by a thin layer of host cytoplasm. Many mitochondria, endoplasmic reticula and vacuoles were observed in the haustorial head, but no nucleus was recognized. Hyphal cell wall of Albugo was observed to continue to the cell wall of haustorial head.