During the 1997-98 El Niño event, the tempera-ture and oxygen contents of the coastal waters off northern Chile were noticeably higher than during non-El Niño con-ditions, but phytoplankton and zooplankton biomasses were not dramatically altered. In contrast, the herbivorous cope-pod Calanus chilensis, which showed a limited tolerance to low-oxygen conditions, exhibited greater abundances, higher growth rates, and a significant reduction in adult body size. These results indicate a positive effect of the changes in oceanographic conditions due to El Niño on zooplankton growth and production, and provide evidence for lack of food limitation on secondary production in the coastal re-gion during this period.