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Summary This work investigated why there are differences in the growth of mussel populations (Mytilus galloprovincialis, Lamarck, 1819) between two different sites in the Kaštela bay. One of the investigated sites is in the central part of the bay (Porat-Kaštel Novi station), while the other is bordered by an open sea (Institute station). The investigation lasted from the 1st of October 2017 until 20th of February 2018. The population density was measured on the 400 cm2 surface area and the population length structure was measured using a vernier scale. Salinity and sea surface temperature were measured according to the GLOBE protocols. The results showed the differences in the number of mussels and the length structure between these two stations, which can be related to environmental factors, salinity and sea temperature. Mussels achieve the best growth when salinity ranges between 20 - 35 ‰, and the sea temperature ranges from 10 ° to 20 ° C. In the central part of the Kaštela bay (Porat station), the density of the mussel population is twice as high as the population measured at the Institute station. Also, the growth and the number of juvenile individuals are higher at the Porat station, due to the inflow of fresh water and thus the more favorable salinity (mean value 35.28 ‰ vs. 37.41 ‰, respectively). Moreover, sea temperatures in the range of 10 - 20°C, high primary production, higher amounts of food and moderate exposure to waves and changes in seawater, make better conditions for growth and reproduction of mussel population in the Kaštela bay.