Mycelial cords are linear aggregations of parallel-oriented hyphae. The mature cords are composed of wide, empty vessel hyphae surrounded by narrower sheathing hyphae. Cords may look similar to plant roots, and also frequently have similar functions; hence they are also called rhizomorphs (literally, "root-forms"). As well as growing underground or on the surface of trees and other plants, some fungi make mycelial cords which hang in the air from vegetation.

Mycelial cords are capable of conducting nutrients over long distances. For instance, they can transfer nutrients to a developing fruiting body, or enable wood-rotting fungi to grow through soil from an established food base in search of new food sources. For parasitic fungi, they can help spread infection by growing from established clusters to uninfected parts. The cords of some wood-rotting fungi (like Serpula lacrymans) may be capable of penetrating masonry.

The mechanism of the cord formation is not yet precisely understood. Mathematical models suggest that some fields or gradients of signalling chemicals, parallel to the cord axis, may be involved.

Rhizomorphs can grow up to 9 m (30 ft) in length and 5 mm (1⁄4 in) in diameter.

Rhizomorphs are a special morphological adaptation root-like structures found in fungi. These root-like structures are composed of parallel-oriented hyphae that can be found in several species of wood-decay and ectomycorrhizal basidiomycete as well as ascomycete fungi. Rhizomorphs can facilitate the colonization of some dry-rot fungi such as Serpula lacrymans and Meruliporia incrassata and cause damage to homes in Europe and North America, respectively, by decaying wood. Another genus that is very well studied for their abundance of rhizomorphs production is Armillaria, with some species being pathogens and others saprotrophs of trees and shrubs.

Known for their role in facilitating the spread and colonization of fungi in the environment, rhizomorphs are the most complex organs produced by fungi. They are made up of highly specialized hyphae that are different in size, orientation, and function. Fungi that possess these structures can compete and grow in harsh conditions.

Rhizomorphs are sometimes called mycelial cords, although they are structurally different: mycelial cords are less complex and have a loose network of hyphae giving an appearance of a fan-like mat, while rhizomorphs are more complex organs that have apically dominant growth tips, water-resistant surfaces, and can transport oxygen. Rhizomorphs and mycelial cords both function in nutrient transport, water absorption, translocation and colonization of substrates.

The development of rhizomorphs begins with a submerged thallus that produces mycelium (hyphae biomass) that when deprived of nutrients and exposed to increasing oxygen, morphogenesis occurs giving rise to pseudo or microsclerotia (survival structures of some fungi), which precede rhizomorph development. Concentrations of oxygen play an important role in the production of rhizomorphs. When there is a high concentration of oxygen in the atmosphere, soil moisture, temperature and pH, rhizomorph production increases.