SUMMARYPseudotetracystis terrestris gen. et sp. nov., a member of the Chlorosarcinaceae, was isolated into axenic culture from dry surface soil collected near Texon in Reagan County, Texas. The characteristics of this presently monotypic genus include vegetative cells with a parietal chloroplast, a single pyrenoid, and polymorphic motile cells which are of theProtosiphon‐type (naked with 2 equal flagella); P. terrestrisis the type species. Vegetative cell division (desmoschisis) forms diads and tetrahedral or planar tetrads, sometimes aggregated in transitory complexes. Reproduction is by the polymorphic motile cells which may function either as gametes to form zygotes which develop without dormancy directly into vegetative cells, or they may function as zoospores and develop directly into vegetative cells without unio