Fungi of the class Oomycetes are generally water molds, a reference to the fact that most species are aquatic. During sexual reproduction, the members of this class form clusters of egglike bodies at the tips of their hyphae. Nearby hyphae grow toward the bodies and fuse with them. Nuclear fusions lead to the formation of sexual spores called oospores that germinate to produce new hyphae.

In the sexual process of reproduction, oomycetes form a unique cell called a zoospore. The zoospore has flagella and is able to move like an animal cell. Certain oomycetes cause downy mildew of grapes, white rust of cabbage, and the late blight of potatoes. Aquatic oomycetes infect fish in aquaria and nature.