a phylum of fungi (kingdom Fungi) distinguished by having zoospores (motile cells) with a single, posterior, whiplash structure (flagellum). Species are microscopic in size, and most are found in freshwater or wet soils. Most are parasites of algae and animals or live on organic debris (as saprobes). A few species in the order Chytridiales cause plant disease, and one species, Batrachochytrium dendrobatidis, has been shown to cause disease in frogs and amphibians.