Meadows of the endemic species Posidonia oceanica (L.) Delile is the largest marine ecosystem of the Mediterranean Sea inhabitings the most developed marine zone of the Adriatic Sea which is characterized by low levels of temperature and salinity fluctuations but also by the constant exchange of sea water. Due to the great diversity of species, the biocenosis of infralittoral algae and the biocenosis of seagrass meadows are the two most endangered biocenoses as a consequences of anthropogenic activities that reduced the total cover of this protected species. The aim of the research is to assess the quality and the level of endangerment of the endemic P. oceanica at fifteen different stations within the southern part of the Istrian peninsula: the protected area of National Park Brijuni and Kamenjak Public Institution, and the unprotected area of Banjole and Medulin. The obtained data will be used for further monitoring of these habitats as well as for the assessment of possible negative impacts that threaten the quality of P.oceanica. The research is also focused on assessing the presence of certain species within the meadows, especially those which are endangered and legally protected. The long-term goal of the project is the active protection of marine species and habitats of the most productive marine ecosystem, which is one of the main hotspots of biodiversity in the Mediterranean Sea. According to the table of categorization of sea meadows, all meadows are categorized as "meadows of very low density" except for the meadows of Polje and Porer which belong to the category of "meadows of low density". Comparing the most common vulnerability factors, it was found that NP Brijuni has the best form of protection.