Many infaunal sandy-bottom communities in shallow waters of the Peruvian upwelling system are inhabited by large coexisting populations of the surf clams Mesodesma donacium and Donax peruvianus as well as by the anomuran mole crab Emerita analoga. Under normal conditions, equilibrium states are possible with any one of these species dominating. A Mesodesma community south of Lima in Peru was investigated over 2, 5 years, covering periods prior to, during and after the El Niño (EN) of 1982–83. It was revisited several times later. Growth, recruitment and mortality and, therefore, production of Mesodesma and Donax varied to some extent before EN. However, during the event Mesodesma became locally extinct and had not recolonized the area by July 1986, three years after the return of normal temperatures. Donax, which took over immediately after EN, never reached the densities of the former dominant Mesodesma. Emerita remained a rare species as well, whereas spionid polychaetes increased in importance. The medium-term effect of the exceptionally strong EN of 1982–83 appears to have been very marked on Peruvian sandy beaches. A comparison is made with other shallow-water communities of the upwelling system, and the importance of EN in terms of oscillations of clam stocks off Peru and Chile is discussed. © 1987 Taylor & Francis Group, LLC.