This thesis analyses the presence and distribution of non-indigenous calanoid copepod species Pseudodiaptomus marinus Sato, 1913 in the port of Šibenik, which is situated in the estuary of the river Krka. Considering the frequent and intensive maritime traffic in the port, it is higly likely that the species was introduced into the Adriatic Sea via balast water that gobally represents one of the main vectors of aquatic organisms’ transfer. Samplings of P. marinus were performed through five sampling occassions, from 2017 to 2019 in the shallow coastal part of the port of Šibenik, from a docked research ship „BIOS DVA“ of the Institute of Oceanography and Fisheries. Apart from the taxonomic analysis of adult male and female specimens, which morphologically matched with the older descriptions of the species, abundance determination, as well as environmental parameters such as temperature and salinity have also been analysed. Recorded abundances of P. marinus were very low (<60 jed. m-3) during the research period, despite the night samplings when the species is presumably numerous in the plankton, because of its complex life strategy; bentic during the day and planctonic during the night. However, we also found adult ovigerous females and copepodite stages (juveniles) in the samples, which is indication of the population establishment in the port. Considering the remarkable adaptibility of P. marinus to unfavorable conditions, it is expected that the spreading of this species will continue in the future. Currently, disturbed relations in the zooplanton community structure and unusual relative proportions between the zooplankton groups haven't been recorded in the port. The focus is on the further monitoring of the port of Šibenik, with the purpose of tracking the invasion of the species P. marinus, along with the introduction of night samplings and the improvements of sampling methodology.