Composition and seasonal changes of nanoflagellates (2-20 μm body size) were investigated from January 1981 until January 1988 at three stations in the Gdańsk Basin. Nine groups of flagellates were distinguished: the Cryptophyceae, Dinophyceae, Chrysophyceae, Prymnesiophyceae, Prasinophyceae, Chlorophyceae, Euglenophyceae, Choanoflagellida, and the group of other heterotrophic nanoflagellates. Total abundance of nanoflagellates at the inshore station ranged from 0.2 to 6.1ּ103 ind./ml (mean of 2.1ּ103 ind./ml), while the biomass varied from 10 to 604 μg/dm3 (mean of 149 μg/dm3). The values were lower in offshore waters. The Dinophyceae, constituting-on the average-30% of the total biomass, Prymnesiophyceae (32% of the total abundance) and Cryptophyceae were dominants at all the stations. The Choanoflagellida as well as the other heterotrophic nanoflagellates contributed significantly to the abundance (11 and 18%, respectively). A succession of the dominant taxa is discussed.