The development of apothecia of Pyrenopeziza brassicae (anamorph Cylindrosporium concentricum) on oilseed rape debris and compost malt agar was observed by scanning electron and light microscopy. On oilseed rape debris, apothecia developed directly beneath the epidermis as small globular structures of dense mycelium, which protruded through the epidermis as they increased in size. The apices of erumpent immature apothecia then developed small ostiolar openings which increased in diameter to expose the hymenia of mature apothecia containing dome-shaped asci interspersed with filiform paraphyses. On compost malt agar, hyphae of one mating type grew towards hyphae of the opposite mating type 3–4 d after inoculation of conidia onto agar surfaces. The subsequent development of apothecia on compost malt agar was similar to that on oilseed rape debris.