The Zygnematales are an order of green algae, comprising several thousand different species in genera such as the well-known Zygnema and Spirogyra. All the members of this group develop into unbranched filaments, one cell thick, which grow longer through normal cell division. Most live in freshwater, and form an important component of the algal scum that grows on or near plants, rocks, and various debris.

Sexual reproduction takes place through a process called conjugation. Here filaments of opposite gender line up, and tubes form between corresponding cells. The male cells then become amoeboid and crawl across, or sometimes both cells crawl into the tube. The cells then meet and fuse to form a zygote spore, which later undergoes meiosis to produce new filaments. Only the female passes chloroplasts on to the offspring, as in higher plants.

The only other group of conjugating algae are the desmids, which live as individual cells often striking for their symmetrical appearance. The two orders are definitely close relatives, and sometimes the desmids are included among the Zygnematales, or the two are grouped in their own division (Gamophyta). Otherwise they fall within the Charophyta, which includes those algae that are closer related to the higher plants than they are to most of the other algae.