An unusual concentration of tens of thousands small fish remains discovered during rescue excavations in the town of Antwerp, Belgium, is described. The material was found in a small depression with no associated archeological material but could be dated to the first half of the 16th century based on its stratigraphic position. About 3500 freshwater fish were found in articulating position and it is shown that they died naturally during a single depositional event after an exceptional flood. The species spectrum and the reconstructed fish lengths make it possible to document the season when the catastrophic mortality occurred. This assemblage differs from the few assemblages of natural mortality reported in the literature, which are all of the attritional type.