The evolution of phytoplankton was studied between December 1980 and August 1985. A total 1269 net and water samples were obtianed in 11 cruises as part of the Estudio Regional del Fen¿meno El Ni¿o--Chile Program covering the area extending from Arica (18¿30'S) to Cha¿aral (26¿20'S) from the coast to 200 n. mi (370 km) westward. In the period which preceded the 1982--1983 El Ni¿o event, the coastal phytoplankton consisted predominantly of blooming diatom species which support a large phytoplanktonic biomass. The cell density up to 20 n. mi (37 km) off the coast was over 100 cells mL-1, with a maximum density nucleus near the coast, where values over 1000 cells mL-1 were found. With the anomalous conditions produced by El Ni¿o in December 1982, changes were detected in the phytoplankton biomass and composition. There was a marked decrease in the biomass, the diatom dominance was restricted to a narrow coast band of 2 to 3 n. mi (3.7--5.5 km), and warm water species of diatoms and dinoflagellates reached the coast. These conditions reached their maximum intensity in May 1983. Phytoplankton started to return to normal conditions in December 1983 with a predominance of large diatoms, which support a biomass somewhat larger than that during El Ni¿o. Small diatoms returned as the dominant species in large blooms in 1985. The cell numbers reached values similar to those during pre-Ni¿o conditions, with a normal neritic and oceanic phytoplankton distribution. Red tides caused by the ciliate Mesodinium rubrum were common during normal conditions before and after El Ni¿o. ¿ American Geophysical Union 1987