The Glomeromycota, or glomeromycetes, are a monophyletic group of apparently low diversity (there are about 200 described species) but tremendous ecological importance. All known glomeromycetes occur in association with plant roots. Most famous are those that form endomycorrhizae. Molecular clock estimates suggest that glomeromycetes diverged from other fungi 600 to 500 million years ago, before the evolution of vascular plants. Establishment of glomerophyte–root symbiosis changed the course of evolution for both participants. Today, most vascular plant species harbor endomycorrhizae, dramatically increasing nutrient uptake from the soil.

Glomeromycetes are hard to study because they cannot be grown independently of their plant partners, and so many unanswered questions remain. For example, no evidence of sexual reproduction has ever been found in this group. Yet genetic studies of glomeromycete populations show some genetic diversity, which is thought to result from parasexual processes. Glomerocytes produce extremely large (0.1 to 0.5 mm diameter) multinucleate spores asexually. These large spores can persist in soils until they come into contact with an uninfected root.