The origin and the formation of the ascospore delimiting membrane in Sordaria humana were studied by electron microscopy. The complete delimiting membrane consists of a double unit membrane system. It originates from tubular elements of 70-100 nm diameter which are formed with a thin membrane 5-6 nm thick and filled with electron translucent materials. The process of development of the delimiting membrane is as follows: Tubular elements first aggregate, and then become arranged in a plane. Soon afterwards, they fuse laterally and become transformed into a delimiting membrane. Spore delimitation starts from the distal end of the centriolar plaque and develops towards the opposite side of the nucleus, preceding the formation of the membrane system.