Marine phanerogam Posidonia oceanica (L.) Delile is an endemic species in the Mediterranean Sea prevalent along the entire coast of the Adriatic Sea. Meadows of Posidonia are the source of organic matter and oxygen in the sea, shelter and home to many species and an essential element for the protection of the coast against erosion. The goal of this research was to determine the main characteristics of Posidonia meadows on three locations under different human impact. So, we measured meadow density and estimated its biomass. On the basis of lepidochronological analysis and leaf biometry we estimated primary production in the researched meadows. The samples of Posidonia used for lepidochronological analysis were collected in April 2011, at 10 meters depth at three locations: one in Zadarski Kanal and two on the island of Dugi Otok - in Zaglav Port and Brbišćica Bay. About twenty shoots were randomly selected and collected from each location. The shoots were counted in situ. The results showed significant differences between three researched meadows in meadow density, leaf area index, biomass and primary production. The meadow density in Zadarski Kanal was 169 ± 28 shoots per m2, in Zaglav it was 275 ± 54 shoots per m2 and in Brbišćica 303 ± 28 shoots per m2. The values of other parameters (leaf area index, biomass and primary production) were also the lowest for the Posidonia meadow in Zadarski Kanal which is under the strongest human impact and the highest for the meadow in Brbišćica Bay which is under the lowest human impact. Leaf area index in Zadarski Kanal was 14951,43 cm2 per m2, 40210,05 cm2 per m2 in Zaglav and 78019,47 cm2 per m2 in Brbišćica. Biomass in Zadarski Kanal was 14,84 g of dry weight per m2, 46,45 g of dry weight per m2 in Zaglav, and 92,29 g of dry weight per m2 in Brbišćica. Estimated annual primary production was from 59,42 to 95,07 g of dry weight per m2 per year for the meadow in Zadarski Kanal, from 250,81 to 357,65 g of dry weight per m2 per year in Zaglav and by far the highest in Brbišćica, from 479,93 to 802,96 g of dry weight per m2 per year. Great differences in leaf area index, biomass and primary production of Posidonia meadows in three locations at the same depth might be connected to human impact.