The results of study of sexual reproduction and life cycles of two new for the flora of Ukraine and two rare species of green flagellate algae are presented. Sexual reproduction of Chlamydomonas sphagnicola (F.E. Fritsch) F.E. Fritsch et Takeda was revealed for the first time. The details of structure of some stages of life cycle and sexual reproduction of Ch. Suboogama Tscherm.-Woess, Ch. Coccifera Gorozh., and Oogamochlamys gigantea (O. Dill) Proschold et al. are clarified. The data on morphology, ecology, and geographical distribution of these species are summarized and completed; original drawings and micrographs are provided as well. The materials concerning taxonomy of studied species with reference to peculiarities of their sexual reproduction and data of molecular phylogeny are discussed.