The Oömycota are related to the brown algae, diatoms, and others, within the Strameopiles. the rest of the stramenopiles are photosynthetic, but it appears that the Oömycota have lost their chloroplasts. Members of this group are important aquatic and terrestrial decomposers and parasites. The potato famine of Ireland, which led to the immigration of many Irish people to North America, was caused by an oömycete, as is the pathogen that is currently attacking oak trees in California.

We will use the genus Saprolegnia as an example of the oömycota. Saprolegnia is commonly called water mold and is widely distributed in streams, lakes, and ponds, and moist soils. While most biologists now recognize this lineage as different from the fungal clade, we will study it here because of its structural similarity to fungi.

Like Allomyces it is aquatic and produces zoospores. These spores, however, are laterally biflagellate. Sexual reproduction is oögamous. Eggs develop within a female gametangium called an oögonium. The male gametangium is called an antheridium. Fertilization occurs within the oögonium and the zygote becomes an oöspore. These are released to regenerate the dominate diploid condition. Rather than the sperm swimming to the oögonium, the entire antheridium and attached hyphae are attracted to the oögonium by secreted hormones. The antheridium attaches to the oögonium and forms a copulation or fertilization tube through which the sperm move into the oögonium. Most species of Saprolegnia are saprobes but one is parasitic on fish and fish eggs.

This structure is an oögonium. The content of a young oögonium is more or less homogeneous. As development continues 1-several thin-walled eggs are differentiated within. After fertilization the zygote develops a relatively thick wall and becomes an oöspore.