One or more cells of Ulothrix develop into zoosporangium under favorable conditions. The protoplast of the zoosporangium divides into 32 protoplasts. Each immature protoplast transforms into a four-flagellated zoospore or cell. Zoospores are of two types. Microzoospores (8-32 per zoosporangium) and megazoospores (1-4 per zoosporangium). Zoospores are pear-shaped and have an eye spot. They emerge from the zoosporangium encased in vesicles. The vesicles then rupture and the zoospores float freely. After 2-6 days of swimming, the zoospores lose their flagella and are enveloped by walls. Later divides to form new Ulothrix filaments.