In Ulothrix sexual reproduction is of isogamous type. Two isogametes of + and - strain come together and fuse as a result a quadriflagellate zygospore is formed which floats on water after sometime it rests on the bottom of the pond at this time its four flagella disintegrate and a wall is formed surrounding it from all sides. After taking rest for a long period this zygote divides meiotically and gives rise to 16 zoospores. These zoospores come out of sac and give rise to the new plants of Ulothrix.