(a) Phycomycetes:
Members of class phycomycetes are found in aquatic habitats and on decaying wood in moist and damp places or as obligate parasites on plants. The mycelium is 
aseptate and coenocytic. Asexual reproduction takes place by zoospores (motile) or by aplanospores (non-motile). These spores are produced in sporangium endogenously. Fusion of two gametes leads to the formation of zygospore which is diploid (2n) in nature. These gametes are isogamous (morphologically alike) or anisogamous (morphologically disimilar). Some members of phycomycetes are Mucor, Rhizopus (bread mould) and Albugo (the parasitic fungi on mustard) etc.

(b) Ascomycetes:
The members of this class are called sac fungi. It is a large group with over 30,000 species include brown, green, blue and pink moulds, powdery mildews, cup fungi, yeasts, morels and truffles. Most of them are terresterial and occur as saprotrophs or as parasites. The parasitic forms cause diseases in plants e.g., powdery mildews, ergots etc. The yeasts are unicellular simplest sac fungi. Except yeasts all ascomycetes are mycelial. The mycelium is well developed and branched. The hyphae are septate and multicellular. The ascomycetes are characterised by complete absence of motile structures in their life cycle. Yeasts usually reproduce asexually by the formation of conidia or conidiospores. They are produced exogenously mostly in chains from the tips and sides of hyphae.

Sexual fusion occurs by gametic copulation (e.g.. yeasts), gametangial contact (e.g., Pyronema), sepermatization (e.g., Ascobolous carbonarius) and somatogamy (e.g., Deziza). Sexual reproduction takes place in three phases plasmogamy, karyogamy and meiosis. Plasmogamy results in the formation of dikaryon consists of dikaryotic cells which act as ascus mother cell. In this cell the two nuclei of opposite mating types fuse (karyogamy) to form diploid ascospores which remain inside the ascus (sac like structure). The asci are club shaped, elongated, ovoid or rectangular in shape. The ascospores are liberated from ascus and germinate to produce new mycelia. Examples are Yeasts, Aspergillus, Penicillium, Neurospora, Claviceps, Morels and Truffles.

(c) Basidiomycetes:
It is a most advanced and largest group of higher fungi with about 1100 genera and 25,000 species. Commonly known as rusts, smuts, mushrooms, puffballs, toad stools, sting horns, bracket fungi etc. Due to presence of large fruiting bodies (basidiocarps) they are the most common fungi. Some produce edible fruiting bodies e.g., mushrooms, some produce extremely poisonous chemical substances. They decompose the wooden materials.

The mycelium of basidiomycetes is composed of filamentous, branched and septate hyphae. The mycelia are of three types:

1. Primary mycelium
2. Secondary mycelium and
3. Tertiary mycelium.

1. Primary mycelium develops from uninucleate haploid basidiospores and consists of uninucleate cells. It multiplies by conidia.
2. Secondary mycelium consists of binucleate cells. It grows with cell division which is accompained by clamp connection to facilate proper distribution of dikaryons. It multiples by the formation of different types of spores such as, chlamydospores, uredospores, teleutospores and a aeciospores etc.
3. Tertiary Mycelium: It is associated with the basidiocarp formation around the basidia. Its hypae are also dikaryotic like the secondary mycelium.

(d) Deuteromycetes:
These are commonly known as fungi imperfecti because only the asexual vegetative phases of these fungi are known. When the sexual forms of these fungi were discovered they were named into classes they rightly belong to. This class include about 1680 genera and 17000 species.
They are saparophytic in soil and grow on decaying matter. Most of them are parasites and cause serious diseases in plants, animals and humans. Hyphae are well  developed, branched and septate. 
Reproduction occurs only by asexual methods by the formation of conidia. The conidia are directly produced on condiophores or in sporodochia, pycnidia, acervuli and synnemata. Sexual reproduction is either absent or not reported. Examples are Alternaria, Colletotrichum, Fusarium, Trichoderma etc.