A light and electron microscopic study on the cell wall and vegetative protoplast of the green alga Apjohnia laetevirens Harvey is presented. Supplementary information on cell wall structure using polarized light and X-ray diffraction and evidence for two types of cross wall formation are also included. The cell wall consists of an amorphous layer, an outer microfibrillar reticulum, numerous inner lamellae of microfibrils oriented in a crossed fibrillar pattern, and a less common layer oriented at 45 degrees to the other fibrillar layers. The parietal cytoplasm contains organelles typical of those found in the Chlorophyta. The filament contains a large vacuole in which are found acicular calcium oxalate crystals. Taxonomic implications of the cytology of Apjohnia are discussed.