In Serbia and Montenegro, four species of crayfish have been identified: three native, i.e., Astacus astacus (river crayfish), Astacus leptodactylus (Danube crayfish) and Austropotamobius torrentium (spring crayfish), as well as one non-indigenous species, Orconectes limosus (American crayfish). According to results presented, which comprise analyses of the data from a 60-year period, A. astacus is the most endangered species of crayfish in Serbia, as well as in Montenegro. The factors that influence the reduction of populations of this species are: pollution, habitat fragmentation, diseases, competition with A. leptodactylus, as well as isolation of the populations. According to IUCN criteria (Version 3.1), the degree of the endangerment. of A..,astacus, in, Serbia is evaluated as "Endangered" ("EN"), which is a higher degree than the, international level determined for this species, i.e., "Vulnerable" ("VU"). Populations the river crayfish in the area of Montenegro are in better condition, especially in the area of the upper course of the Zeta River. However, the populations of river crayfish in Montenegro are endangered by overfishing. Among native crayfishes, the least negative changes were determined in populations of A. torrentium. In both countries, the status of this species, according to IUCN criteria, was designated as "Lower Risk/Near Threatened" ("LR/nt"), which is lower than the international level of this species: "Vulnerable". During the past 30 years, A. leptodactylus has extented its area of distribution from the Danube River to other waters in Serbia (reservoirs and the Morava River). The last five years, the Danube crayfish has been endangered by the penetration of the invasive American crayfish. The abundance ratio of these two species in the Djerdap stretch of the Danube River was estimated as O. limosus: A. leptodactylus = 5: 2.