Paralia sulcata is a tychopelagic centric diatom species common in the North Sea. Due to the fact that P. sulcata is found both in the sediment and the water column, it is assumed to be an important food source for both benthic and pelagic grazers. We know a little about the environmental conditions associated with the occurrence of P. sulcata, but almost nothing is known of its ecological role. Thus the aim of this study was to investigate the ecology of P. sulcata. The Helgoland Roads long-term data set (North Sea), in which phytoplankton and physicochemical parameters have been sampled since 1962, served as the environmental data set. To detect possible species-environment relationships, multivariate statistical analysis was carried out (canonical correspondence analysis). Annual niche breadth and niche position (outlying mean indices) were calculated for P. sulcata. Up to 1996, P. sulcata occurred mainly in late autumn and winter, but since 1997 it has been found throughout the year. The niche position and niche breadth for P. sulcata varied over the sampling period. There was a change in the ecological niche of P. sulcata, with a shift from a more specialized to a more generalized niche and a new occurrence of this diatom in summer during the last 10 yr. Changing temperature, light and nutrient conditions at Helgoland Roads could be responsible for the new occurrence and the shift in the ecological niche of P. sulcata. © Inter-Research 2009.