Many phytoplankton species are susceptible to fungal parasitism. Parasitic fungi of phytoplankton mainly belong to the Chytridiomycetes (chytrids). Here, we discuss the progression made in the study of chytrids that parasitize phytoplankton species. Specific fluorescent stains aid in the identification of chytrids in the field. The established culturing methods and the advances in molecular science offer good potential to gain a better insight into the mechanisms of epide-zic development of chytrids and coevolution between chytrids and their algal hosts. Chytrids are often considered to be highly host-specific parasites, but the extent of host specificity has not been fully investigated. Chytrids may prefer larger host cells, since they would gain more resources, but whether hosts are really selected on the basis of size is not clear. The dynamics of chytrids epidemics in a number of studies were partly explained by environmental factors such as light, temperature, nutrients, pH, turbulence and zooplankton grazing. No generalization was made about the epidemic conditions; some state unfavorable conditions for the host growth support epidemic development, while others report epidemics even under optimal growth conditions for the host. Phytoplankton is not defenseless, and several mechanisms have been suggested, such as a hypersensitivity response, chemical defense, maintaining a high genetic diversity and multitrophic indirect defenses. Chytrids may also play an important role in food webs, because zoospores of chytrids have been found to be a good food source for zooplankton.