ABSTRACT. – The physicochemical characteristics and planktonic taxa of ballast waters were analyzed in 6 ballast tanks from vessels that had arrived from different ports around the world.The ballast tank water had been held for 4-30 days. Phytoplankton was the dominant group in all samples and its abundance was highly variable. The Mediterranean sample recorded the highest abundance with 77 10³ cell.L^-1, whereas the Black Sea1 sample recorded the lowest abundance with 5680 cell.L^-1. This group was dominated by diatoms: 57 % in the first sample from the Black Sea (BL1), 91 % for the second sample from the Black Sea (BL2), 80 % for the sample from the Indian Ocean (IO), 58 % for the sample from the Mediterranean Sea (MS), 80 % for the sample from the Baltic Sea 2 (B2) and 56 % for the sample from the Baltic Sea 1 (B1). Other taxa were poorly represented in all samples. Taxonomic identification revealed nine potentially harmful species. Zooplankton was weakly represented in most samples. The thecamoeban protozoan Arcella sp. was represented in all samples, except for one (MS) which comprised 44 % copepod nauplii and 25 % Oithona nana.