The impact of pollution on the vertical and horizontal distribution of microplankton in Izmir Bay (Aegean Sea). The microplankton composition, distribution and the environmental parameters in which affected on the planktonic community of the bay of Izmir were investigated qualitatively and quantitatively between the years 1998-1999 on the seasonal basis. The one-celled planktonic community structure was examined at 14 stations. At the end of the studies a total of 210 taxa were determined from the class Cyanophyceae, Dinophyceae, Dictyochophyceae, Bacillariophyceae and Ciliata. The classes of Dinophyceae and Bacillariophyceae are dominant over other classes in respect to species diversity and cell density. An increase in phytoplankton cell density is observed at the inner Bay, the shallowest part of Izmir Bay, especially due to terrestrial influence and eutrophication is prominent at this part because of low water circulation when compared to other parts of the bay.