Against a background of controversy concerning the absolute magnitude of biological productivity in the oligotrophic, tropical ocean1–4, evidence is accumulating for the existence there of a population of minute, unicellular organisms collectively known as picoplankton5,6. In the tropical Pacific, it has been estimated recently7 that cells passing a 1-µm screen (picoplankton) make a very substantial contribution to the rate of turnover of phytoplankton biomass. On the other hand, previous work in the tropical Atlantic8 led to the conclusion that particles in the size class ≲3 µm are fragments of larger cells and metabolically inert. We present here the first data on the photosynthetic characteristics of picoplankton collected at sea. This new evidence from the tropical North Atlantic supports the argument that the picoplankton contains a significant, metabolically-active, autotrophic component, capable of supplying about 60% of the total primary production hi an open-ocean ecosystem.