Mesoscale physical and biological measurements were made in a cool upwelling filament in the Benguela Current system, off Namibia in February 1988. The filament had been visible on a satellite infra-red image 2 weeks prior to the survey, and was found to be in a decaying state. Temperature, salinity, 0-50 m integratred nitrate, ammonium, oxygen and plankton data have enabled a consistent picture of the structure to be formed, involving four different water bodies. These are (1) typical continental shelf upwelled water inside the shelf break, (2) filament water characterized by salinity of 35.2, high surface integrated nitrate, small phytoplankton cells, and large numbers of salps, (3) warm, blue oligotrophic water to the north, and (4) a warm, high salinity anticyclonic eddy to the south. It is suggested that this southern eddy may have originated from the Agulhas Current near Cape Town. Anomalously low nitrate concentrations, that could not be explained, were found in a 100 m thick layer between 100 and 200 m depth on the southern edge of the structure.