Changes in the community structure and distribution patterns of 71 species of copepods collected during eight oceanographic surveys at Bahia Magdalena, Mexico were analysed. From March 1983 to January 1985, the responses of the copepod community structure to the seasonal environmental conditions and temporal changes during the peak warming of the El Nino event 1982-83, the relaxation phase in 1984, and the winter 1984-85 cooler period were studied. As in other estuaries and bays, the copepod community had a few dominant species, which have a seasonal succession throughout the year related to seasonal changes in sea-surface temperature. There are two major successional stages in the annual cycle of zooplankton development. During the warming period (July October of 1983 and 1984), Acartia lilljeborgii Giesbrecht and Acartia tonsa Dana (tropical affinity) were the dominant species in the copepod community. During the cooler period (December-June), the dominant species was Paracalanus parvus (Claus). During Summer and Autumn 1983 (El Nino peak), an unusual increase of tropical-equatorial copepod species were found, that are regularly found offshore. The resident species Acartia clausi Giesbrecht was replaced by A. tonsa. Despite these changes in the species composition because of El Nino, seasonal succession is the general rule in Bahia Magdalena and was only enhanced by the presence of several tropical species during the warmest period.