El Niño caused physical and biological changes in the northern anchovy habitat off southern California. Anomalous sea surface temperatures, surface currents, mixed layer depths, and plankton biomass levels began to appear in late 1982 and persisted into 1984. Growth of juvenile and adult anchovy slowed during El Niño, probably due to reduced availability of zooplankton prey. A decrease in size-at-age in early 1983, with a recovery in late 1984, can be explained by movements of the stock and the latitudinal cline in size-at-age. Spawning range expanded in 1983 due to shifts in sea surface temperature boundaries. Early larval mortality was unusually high in the yolk-sac stage. Fecundity per unit spawning biomass was low in 1983, due primarily to a high proportion of first-year spawners. Size-at-age was very low by spring 1984, but specific fecundity was surprisingly high. Although El Niño had a variety of significant effects on the northern anchovy, the stock seems to have recovered in 1985.