The smallest and simplest fungi, chytrids are found in aquatic or semi-aquatic environments such as oceans, lakes, rivers, or even damp soil. They feed on living or decaying organisms, and are important decomposers. Their spores have flagella for enhanced movement in aquatic environments.

The first chytrids were found in northern Russia, not long after the Precambrian period. The age of the chytrids, along with the resemblance of certain chytrids to modern groups (Rhynie chytrids resemble the Spizellomycetales, Devonian chytrids resemble the genus Allomyces), support the idea that the chytrids are the ancestors to all members of the kingdom Fungi. For this reason, the chytrids give scientists a look into what the first fungi looked like.

Chytrids, as stated above, are mainly aquatic. They have flagellatd gametes (helpful to propel in water), and have chitin that strenghthen the cell walls. One group is unique among fungi in having cellulose, the main component of plant cell walls. All of the above are uniting factors to the chytrids, as well as hallmarks that were probably present in the ancestor of all fungi divisions. Some of these, like flagellated sperm, have been lost in all other groups, but other features, like a chitin cell wall, still unite the fungi.

Division Chytridiomycota is a very diverse group. The organisms in this division live in salt or freshwater, are parasitic or decomposers, and can be unicellular or very advanced, producing a mycelium like other fungi. Few have importance in our lives, except for a type that produces potato wart, and others that can be used in research.