The research conducted from March 12, 2024 to May 28, 2024 included a comparison of the biodiversity of snails of the subclass Heterobranchia at two locations in the northern Adriatic, Valbandon (VB) and Mali Portić (MP), which differ in the degree of anthropogenic influence. A total of 12 species were recorded, of which nine species were found at the VB location, and seven species at the MP location. The total abundance and density of organisms was higher at VB, while a higher Shannon index was recorded at the MP location, suggesting greater biodiversity. The dominant species Aplysia punctata in the VB is a potential bioindicator of heavy metal pollution. The consequences of anthropogenic impact on the VB location are reflected in the composition of species, their abundance and biodiversity. On the other hand, the MP was dominated by the species Elysia timida, whose population growth was associated with the appearance of the alga Acetabularia acetabulum. The species Thuridilla hopei showed an increase in abundance with increasing temperature. The presence of the thermophilic species Trinchesia miniostriata in VB indicates the potential impact of climate change on the expansion of the range of this species. The research results highlight the importance of further monitoring and research in order to gain a more comprehensive insight into the biodiversity of snails of the subclass Heterobranchia in the northern Adriatic.