Ascospores and septal structures of both vegetative and reproductive hyphae of Leucangium carthusianum were studied ultrastructurally. The apiculate ascospores contain a couple of large guttules and multiple nuclei (more than four). Early spore wall development is similar to that of most epigeous and hypogeous Pezizales studied. The mature spore wall has a very thick primary wall with a bricklike arrangement of wall fibers and a three layered epispore. Septal structures of vegetative hyphae are constructed of laminate pore plugs and globose, hexagonal, and cylindrical Woronin bodies, which are typically found in the Pezizales. In reproductive hyphae. septal pore plugs are dome shaped with central laminations. Cylindrical Woronin bodies are found around the septal pore in ascal bases. Using ultrastructural data, we find that L. carthusianum is closely related to the Morchellaceae and Helvellaceae. However, the placement of L. carthusianum in either family is not certain.