Photosynthetic prokaryotic picoplankters (<2µm), Synechococcus spp.1,2, provide a significant fraction of oceanic primary production3,4. The significance of similarly sized eukaryotic algae has not been fully appreciated until recently, as they are difficult to isolate in culture and many become distorted or disintegrate in preservatives5,6. Yet these eukaryotes, in the size range of Synechococcus, to perhaps 10 µm (refs 7–10), often numerically dominate ultraphytoplankton (<10µm) at deep chlorophyll maxima5,6,11–13. Because temporal variability in oceanic productivity seems to be caused by changes at the bottom of the euphotic layer14, the composition and activity of communities at chlorophyll maxima are of interest. Here we present evidence that eukaryotic ultraplankters have greater photosynthetic and growth efficiencies than Synechococcus in the dim blue–violet light occurring at the bottom of the euphotic zone. Our experiments at least partially explain the large numbers of eukaryotic ultraplankters in the open ocean at the 0.5% light level, below the population peak of Synechococcus spp.