Non-native alien zoopathogens have had a profound impact on the health of aquatic zoobiota in Republika Srpska and the Republic of Serbia as well as around the region and the world as a whole. The movement of disease-causing microfungi around the world has taken its toll on many different organisms, and continues to intensify with an increase in global transport of cargo, people, and animals. Those who study fungi have little information about the characteristics of invasive, zopathogenic fungi because of the fact that a particular fungus can inhabit many different organisms, and may be pathogenic to some of these hosts but not to the others. Crayfish plague (Aphanomyces astaci) broke out through the Danube River to the territory of Bačka and Banat (today A.P. Vojvodina, the Republic of Serbia) in 1879. Until 1880, the disease spread through the Sava River to the northern Bosnia (today Republika Srpska). From 1955 until 1970, almost all territories of Republika Srpska and Republic of Serbia were afflicted with the crayfish plague disease. Despite the fact that the problems of the spread of crayfish plague in the territory of the Republika Srpska and the Republic of Serbia have been of great significance, there is little data available for analyzing this issue. According to IUCN criteria, and mostly due to the Aphanomyces astaci invasion, the degree of endangerment of noble crayfish in Serbia was evaluated as “Endangered”, which was a higher degree than the international level determined for this species, i.e., “Vulnerable”