Although representatives of all fungi are to be found in the aquatic habitat, the most abundant in terms of numbers of species are the aquatic Phycomycetes. In planktonic and benthic communities of the lacustrine ecosystem chytridiaceous and saprolegniaceous fungi have significnat roles as parasites of algae and decomposers of bottom detritus. In the case of epidemic fungal infections of planktonic algae, activities of fungi may affect the composition of phytoplankton communities by delaying the time of algal maximum and by reducing the population of certain algae so that other phytoplankters will replace the infected algal populations. In the case of other infections that are not epidemic the fungi may not influence populations of algae during periods of maximum algal population. Instead, the fungi may only infect phytoplankters during periods of decline in the algal population and thus only hasten the decomposition of the plants. Some chytridiaceous fungi grow only on algal cells that are obviously dead. The foregoing indicates that there are probably three situations with regard to the relationship of these fungi to algae in the plankton: (1) Fungi may be obligately parasitic attacking living algal cells during periods of active growth; (2) fungi may not be obligately parasitic and only attack algal cells in a senescent condition during periods of decline in the algal population; and (3) the fungus lives only on dead algal cells. There appears to be a pattern to the distribution of chytridiaceous planktonic fungi that infect diatoms at various depths in a lake. For the most part the oldest structures in the fungal life cycles are found at the greatest depths, indicating that fungus infections may occur near the surface and that the life cycles of the fungi proceed as the diatom cells drop to the bottom. Aquatic Phycomycetes are commonly found in the benthic community. Fungi most commonly found during studies of Douglas Lake, Michigan, were members of the Chytridiales that decompose chitin. Thus these organisms may play a significant role in chitin decomposition in this lake. However, studies in Lake Michigan where the bottom depths were greater indicated the oomycetous fungi were the most common.