Some features of the ultrastructure of Volvox are described. Golgi bodies were often associated with the endoplasmic reticulum (ER), and the two basal bodies appeared to be accompanied by two probasal bodies. A few vegetative cells were binucleate. All cells examined had a peripheral cytoskeleton of microtubules which was particularly well developed in the cells of sperm packets. During inversion of a colony, the cells elongated considerably, possibly due to the increased length of these peripheral microtubules; the cell profile also became some-what narrowed at the inner edge of the flexing colony. Cytoplasmic connections were large and numerous in young coenobia, but were generally absent in older vegetative colonies; by inversion, they had become confined to the chloroplast end of the cells where they seemed to act as hinges. Elements of the ER ran through these interconnections, possibly providing an intercellular communication network needed for the coordinated activity of inversion. A new structural feature was discovered in the form of circular (or possibly spiral) striations on the plasmalemma around these cytoplasmic connections. They were detectable just before inversion, and were most pronounced immediately after.