Plankton refer to free-floating organisms suspended in the water column that have little or no resistance to water current. Within this study, emphasis is placed on a plankton size fraction between 20-200 microns or microphytotoplankton and microzooplankton. The main objectives of this thesis are: (i) to investigate composition, abundance and distribution of microplankton in South Adriatic Pit, (ii) to determine the differences in the abundance of microplankton groups between the studied sites and (iii) to explore the relationship between physico-chemical parameters and the spatial distribution of diverse microplankton groups. The study was conducted in South Adriatic in the period from 28 February to 3 March 2015 at 13 stations along two transects. Samples were collected using plankton nets and Niskin samplers and analyzed using Zeiss Axiovert 200 and Olympus IMT-2 inverted microscope in order to obtain data on the taxonomic affiliation and abundances of individual microplankton groups. The results showed that there is no great difference between the sampled stations regarding the composition of microphytoplankton and microzooplankton. Different microphytoplankton and microzooplankton groups showed preferences toward different environmental conditions primarily to the nutrient concentrations which is then reflected in their distribution in the water column. The microphytoplankton community was strongly dominated by diatoms found up to 500 m of depth. The results showed 131 different microphytoplankton species. The highest abundances of microzooplankton in the open waters of the Southern Adriatic Sea during the winter period were recorded.