Allomyces macrogynus is a member of the Chytridiomycetes (Blastocladiales). The Chytridiomycetes represent one of the four classes of the Eumycota ("true fungi") and contain the most ancestral fungal lineages ("lower fungi"), namely Blastocladiales, Chytridiales, Monoblepharidales and Spizellomycetales. Their phylogenetic branching order has not been unambiguously defined, neither through the use of ribosomal sequences nor by ultrastructural analysis.
The filamentous cells of Allomyces grow essentially without limitation in length, and divide in a characteristic dichotomous pattern that results in a perfectly geometric mycelium, on solid media. Hyphae (filament segments) are separated by pseudosepta that allow free circulation of intracellular materials along the filaments. The reproductive structures, which are located at the end of the hyphae, are isolated by complete septa. Allomyces is most commonly found in humid soil within the tropics, although it has been identified in many different regions of the world. Its life cycle can alternate between two stages, gametophytic and sporophytic, reflecting its capacity of both sexual and asexual reproduction. Its asexual cycle (like that of all other Chytridiomycetes) is perpetuated through the formation of flagellated zoospores which survive long time periods in water, and which are able to move very efficiently in liquid media.
Chytridiomycetes have been traditionally included within the group of "zoosporic fungi" that are also refered to as "aquatic fungi". As discussed in the phylogeny section, the zoosporic fungi represent an artificial group of unrelated organisms (are polyphyletic). Certain 'zoosporic fungi' like the Oomycetes are clearly associated with the Stramenopiles (including golden and brown algae and diatoms, amongst others), considering ultrastructural features and phylogenetic analyses.
For a more detailed description of Allomyces, see the review of Olson (1984), which describes many details of its life cycle, light microscopic and ultrastructural views, genetics, basic experiments in teaching, etc..