The ecological conditions of Mali Ston Bay are suitable for the settlement of the European flat oyster Ostrea edulis Linnaeus, 1758, which has been cultivated in this area for centuries, and the bay of Bistrina is cited as one of the most productive parts. The traditional method of collecting spat was based on submerging the branches of oak or spruce. Currently, plastic caps strung on iron profiles and other forms of plastic collectors are used. Given that the substrate immersed in the sea represents a new habitat, in addition to oyster fry, the collectors are inhabited by a number of different organisms that can have a positive or negative impact on the growth and development of oysters. The aim of this research was to identify the organisms that inhabit the collectors for the collection of fry in Bistrina bay and their mass. For the purposes of this research, nine triplicates of collectors (plastic caps strung on iron profiles) were installed in the bay of Bistrina for a period of nine weeks. The first triplicate was installed on April 6, 2022. After a little more than four months of exposure to the settlement, triplicates were removed, weighed and analyzed, and the fouling organisms were determined. The first extraction and analysis of the collectors were carried out on August 30, 2022., after which one triplicate was analyzed per week. A total of 41 different types of organisms were recorded, with four taxa determined to the genus level and two to the genera to which they belong. Some non-native species, such as Paraleucilla magna, Hydroides elegans and Amathia verticilata, were recorded among them. Differences were observed in the total and average mass of the samples depending on the side of the caps from which they were taken; the fouling process is more intense on the underside of the caps. The only organism found in each sample was the serpulid tubeworm Hydroides elegans Haswell, 1883.