In this paper, the state of the coralligenous biocenosis and associated species is monitored at 8 different stations in the Mljet National Park. The diversity of species at the researched stations of the coralligenous biocenosis is quite large, and 415 species of algae and animals were determined at the researched stations. The highest similarity among species was determined between the stations Rt Lenga and Vranji Škoj (95.75%), Rt Lenga and Velika priveza (95.14%), and Hrid Štit and Korizmeni Rat (94.98% similarity each). The coralligenous biocenosis is widely distributed along the eastern coast of the Adriatic Sea, primarily on the outer islands, and is strongly negatively influenced by increased sea temperatures due to global climate change. Sea temperatures measured at the Rt Lenga station at depths of 30 to 50 meters show a trend of temperature anomalies during the summer months. The most endangered are the stony corals Madracis pharensis and Leptopsammia pruvoti, and the red coral Corallium rubrum. Populations of red coral (C. rubrum) are also threatened by illegal fishing and excessively high sea temperatures at depths below 40 meters. The coral Leptopsammia pruvoti has a high percentage of dead individuals at the research stations (over 30%). The coralligenous biocenosis in the seabed of the Mljet National Park is well preserved, with less negative anthropogenic influence (primarily fishing tools and sea pollution). The main problem, however, is climate change, which causes temperature anomalies at greater depths (between 30 and 50 meters deep). Invasive species also pose a threat to the coralligenous community, and the biggest problem is currently represented by Caulerpa cylindracea, Hermodice carunculata and Sparisoma cretense. At the researched stations, fishing nets were found that damage sessile coralligene species (algae and animals) and irreversibly destroy the species.