AbstractSeasonal variations in the efficiency of utilization of radiant energy by phytoplankton photosynthesis in a tropical embayment followed the same pattern as that of primary production. showing that incident radiation did not limit phytoplankton production. Peak values in primary production and photosynthetic efficiency coincided with periods of nutrient enrichment caused either by upwelling during the southwest monsoon or by run‐off during the northeast, monsoon. A world‐wide comparison of published data on photosynthetic efficiencies in different latitudes showed that higher efficiency is achieved in the tropics, where the values are similar to those observed on phytoplankton cultures. Based on the maximum efficiency of photosynthesis attained, three regimes can be recognized in tropical waters: 1) regions of upwelling with a maximum of 6%,. 2) coastal bays with 0.7%, 3) oceanic regions with 0.