The freeze-etching technique, aldehyde fixation and heavy metal shadowing of wall material were used in an electron microscope study of the maturing spores of Alternaria brassicicola (Schw.) Wiltshire. The walls are composed of fibres, probably of chitin. The plasmalemma has rectangular grooves in its outer surface and corresponding ridges on the inner one; both surfaces bear particles of two distinct sizes. Endoplasmic reticulum may be lamellated or vesicular and its involvement in wall formation is confirmed; vesicles produced by the endoplasmic reticulum fuse with the plasmalemma. The structure of nuclei, mitochondria and vacuoles is also demonstrated.