Like the chytrids, the oomycetes are called water molds. Vegetative growth consists of non-septate (cross-wall lacking) filaments. Cellulose is the prominent component of filament walls, a characteristic that stands in contrast to the chitinous walls of other fungal groups. Many oomycetes grow on decaying on decaying plant and animal matter. Others are parasites. The group includes some very devastating plant pathogens like potato and grape blights.

The name of the group is derived from the production of large eggs in an enlarged female gametangia. The technical term for these gametangia is oogonia. (Fig.1) Fertilization of eggs is accomplished by the release of sperm nuclei into an oogonium from a filament that surrounds the structure. Both eggs and sperm are the products of meiotic events in gametangia. The vegetative filaments are diploid with respect to the number of chromosomes in their nuclei. The zygospores produced by fertilization are also diploid. Gametes of most water molds are not flagellated. However, flagellated cells are routinely produced asexually at the tips of vegetative filaments. (Fig.2) Asexually produced flagellated cells are called spores. There are two flagella on the anterior end of each spore, one whiplash and one "tinsel." In the Saprolegnia (one of the most common freshwater oomycetes) the asexual spores can lose their flagella and "encyst." Germination of an encysted spore gives rise to a secondary spore with lateral insertion of flagella. The flagellated spores of Saprolegnia are capable of repeated encystment and emergence if conditions are not suitable for growth. Sexually produced zygospores are also capable of surviving adverse conditions.

There are 700 species of oomycetes. Good fossils of filaments and spores that closely resemble Saprolegnia have been found in the middle Devonian period, at about the time that terrestrial organisms were beginning to evolve. Chemistry, morphology and molecular data allay this group with later diverging heterokonts (i.e., diatoms and brown algae).