The order Ulotrichales is characterized by features both of the vegetative structure and of the life cycle of their members. The thalli are simple filaments in Ulothrix and Urospora, prostrate discs or monostromatic blades in Gomontia and Monostroma. Monostroma is restricted to forms that develop from saccate or filamentous primary stages, never such ones that arise as tubular growths upon a prostrate disc. In general, isomorphic generations are connected by zoospores. In addition, the new concept of the order unites the majority of examples of heteromorphic life cycles in the Chlorophyceae. Their members agree in the possession of a unicellular cyst-like generation, formerly known as Codiolum. Elsewhere in the Chlorophyceae, heteromorphic life cycles occur sporadically, as in Derbesia (Siphonales), Spongomorpha (Cladophorales), and Stigeoclonium (Chaetophorales).