Minimum daily growth rates of Prochlorococcus were estimated for the central equatorial Pacific (12 ¿S--12 ¿N, 140 ¿W) during El Ni¿o (February--March 1992) and normal upwelling (August--September 1992) conditions. Growth rate estimates were based on the percentages of cells in the S and G2 division phases at dawn (~0700 LT) and dusk (~1800 LT) as approximate values for fmin and fmax, respectively. During both environmental conditions, depth-integrated growth rates of Prochlorococcus were higher in oligotrophic waters at the northern end of the transect (0.36--0.52 d-1) and decreased to a minimum at the equator. The lowest growth rates were found at the equator during El Ni¿o and at 2 ¿N during normal upwelling, where a large biomass of buoyant diatoms had accumulated in the vicinity of a convergent front. Prochlorococcus growth rate reached a high of 0.64 d-1 at 1 ¿S and maintained a moderate rate (0.36--0.49 d-1) throughout the southern end of the transect. An inverse relationship was found between the contribution of Prochlorococcus to the total primary production and nitrate concentration as well as total primary production measured by the 14C method. While Prochlorococcus is a dominant primary producer in tropical and subtropical open-ocean ecosystems generally, it is relatively more important in oligotrophic waters than in the nutrient rich equatorial upwelling zone. ¿ 1999 American Geophysical Union