Many ocean ecosystems ar ethought to be heading towards a domiance of gelatinous organisms. However, gelatinous zooplakton has been largely understudied and the absence of quantitative long-term data for the studied area impeded drawing conclusion on the potential increasing densities. This study gives a comprehensive overview of the spatio-temporal distribution patterns of gealtinous zooplankton in terms of diversity and density bewteen March 2011 and February 2012 in the Belgian part of the North Sea and the adjacent Westerschelde estuary, based on monthly and seasonal samples respectively. ... : The gelatinous zooplankton samples were collected using a 1m-diameter, 1000µm-mesh CalCOFI plankton net, undulated three times through the water column (sine wave from sea surface to bottom) at a speed of 3 knots. One replicate was taken at each station. In the Belgian part of the North Sea, three stations on a coastal-intermediate-open sea transect were monthly sampled. Additonally, seasonal samples (March, June, September and December) were taken at three stations in the Westerscheldt estuary from the lower over the middle to the upper estuary. Most Ctenophora and Scyphozoa were isolated fro mthe CalCOFI samples on board and immediately identified, counted and measured (oral-aboral length and disch diameter, respectively; mm). The rest of the samples was preserved in a 4% formaldehyde-seawater solution. The fixed samples were further analysed in the laboratory, where the hydromedusae and the remaining small ctenophores and scyphomedusae were isolated, counted and identified to species or higher taxon level. ...