The phylogenetic relationship and origin of the euglenoids are controversial at present. It is not clear which of the extant genera may be most primitive, although Distigma proteus has been suggested as an early progenitor by some researchers. Scanning (SEM) and transmission (TEM) electron microscopic data were obtained in an effort to clarify some of the major taxonomic characteristics of this organism. SEM analyses of cells fixed during euglenoid movement (metaboly) show that the spirally arranged pellicular strips in the expanded regions of the organism have a lower pitch than those in the more constricted regions. This finding reveals the mechanical basis for euglenoid contortional movements. Mitochondria observed by TEM contain discoidal cristae, but some mitochondria are particularly large (e.g., 1.5 μm) and contain concentrically arranged, multiply layered cristae located deeper within the matrix, as in Khawkinea sp. Acid phosphatase reaction product is located in the cisternae and peripheral saccules throughout the Golgi stack and is regularly observed in the cisternae of the ER located beneath the pellicular ridges. Vacuoles varying in size, containing acid phosphatase reaction product, occur particularly near the periphery of the cell including the region surrounding the flagellar pouch. Occasional deposits of the reaction product within what appear to be membranous extrusions occur at the outer surface of the cell and may be secreted from sub-pellicular organelles. © 1994 Wiley-Liss, Inc.