Plant–parasitic green algae in the genus Cephaleuros infect leaves, twigs and fruits of numerous host plants worldwide. The reproductive structures of Cephaleuros are important in the infection process. The goal of this study was to determine the in vitro morphology and behavior of the zoospores and gametes of five Cephaleuros species: Cephaleuros karstenii, C. pilosa, C. solutus, C. virescens and Cephaleuros sp. Microscopic observations revealed that zoospores were ellipsoidal, rod-shaped, or spherical with four flagella. Gametes were spherical in shape with two flagella. Zoospores were released from all five Cephaleuros species, but gametes were released only by C. karstenii, C. solutus and Cephaleuros sp. After their release from gametangia, gametes swarmed in a water drop in irregular and circular motions until the resting stage; some gametes attached to each other, and others burst. Zoospores were released from papilla-pores located at the base of zoosporangia and swarmed in a water drop in irregular and circular motions. Some zoospores germinated and others burst, similar to the gametes. In this study, germinated zoospores formed germ tubes, and filaments containing carotenoid pigment, and then died without forming thalli.