Two different types of appendages in Chlorococcales have been investigated. (1) The bristles of some species of Scenedesmus and of Polyedriopsis have the same appearance as that described earlier for some other Scenedesmus species. In contrast to other authors we believe that the regular pattern is caused by the projection of interlocked sinusoidal proteinaceous microfilaments. The bristles of Micractinium and Pediastrum are likewise composed of similar sinusoidal microfilaments, which, however are arranged in two different ways. (2) The spikes of Siderocystopsis consist of—probably cellulosic—banded microfibrils which are composed of about 2-nm-thick subfibers and end perpendicular to the surface of the cell wall. Acanthosphaera has a very complex cell wall. The spikes consist of about 3- to 4-nm-thick microfibrils which are embedded in a dense matrix and end nearly parallel to the surface of the cell wall. In addition, Acanthosphaera bears spicules on the spikes and on the compact part of the cell wall. Presumably, self-assembly plays an important role in the formation of these appendages.