The fine structure of vegetative mycelia of the filamentous Ascomycete, Neurospora crassa, has been investigated by the standard techniques of electron microscopy. Addition of uranyl nitrate to the methacrylate-embedding medium minimized disruption of the specimens—an accident often observed in the preparation of microbial material.

This report describes the presence of a chitinous polysaccharide wall containing fine fibrils embedded in a homogeneous matrix. A sinuate plasma membrane lies adjacent to the inner wall surface. This membrane is often closely associated with the endoplasmic reticulum of the cytoplasm. Numerous mitochondria of the classical type, and dense particles of 10 mµ diameter occur throughout the cytoplasm. The nuclear region is surrounded by a double membrane with pore openings. Associated with the nuclear envelope is a dense area, the nucleolus.

The significance of these observations and their relationship to other forms is discussed.

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