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In order to assess the influence of the River Po outflow on microphytobenthos distribution, two areas of the northern Adriatic Sea, differently affected by the River Po outflow,were investigated. Sediment samples were collected in June 1996 and February 1997 from two grids of six stations (from 14 to 66 m deep) located across the River Po plume front. In the top 1 cm of sediment, chlorophyll­α ranged from 11.2 to 278.8 mg m­2, often exceeding integrated values in the water column, except during the winter phytoplankton bloom. Microphytobenthos densities (size >20 µm) ranged from 728 to 9278 cells cm­3. Diatoms largely dominated microphytobenthic assemblages with both epipelic motile (Pleurosigma spp., Gyrosigma spp., Navicula spp.) and non­motile forms (Paralia sulcata,Hyalodiscus laevis).Filamentous cyanobacteria (Oscillatoriales) were abundant in the summer.The vertical distribution of microphytobenthos revealed that a considerable fraction of the microalgal biomass (up to 40%) was located in the 3–6 cm sediment layer. Microphytobenthos did not display significant changes between sampling periods. Coastal sediments of the northern area displayed high densities of the marine planktonic Thalassiosira angulata, suggesting that high sedimentation rates occurred in the area facing the River Po delta. Conversely, in the southern area pennate epipelic diatoms dominated the microphytobenthic community. In both areas, offshore sediments displayed the dominance of the centric benthic diatom Paralia sulcata. The results of the present study indicate that the plume, which determined marked coast­offshore gradients, influenced both abundance and community structure. [ABSTRACT FROM AUTHOR]