Picoplankton--cells with a diameter of less than 3 microm--are the dominant contributors to both primary production and biomass in open oceanic regions. However, compared with the prokaryotes, the eukaryotic component of picoplankton is still poorly known. Recent discoveries of new eukaryotic algal taxa based on picoplankton cultures suggest the existence of many undiscovered taxa. Conventional approaches based on phenotypic criteria have limitations in depicting picoplankton composition due to their tiny size and lack of distinctive taxonomic characters. Here we analyse, using an approach that has been very successful for prokaryotes but has so far seldom been applied to eukaryotes, 35 full sequences of the small-subunit (18S) ribosomal RNA gene derived from a picoplanktonic assemblage collected at a depth of 75 m in the equatorial Pacific Ocean, and show that there is a high diversity of picoeukaryotes. Most of the sequences were previously unknown but could still be assigned to important marine phyla including prasinophytes, haptophytes, dinoflagellates, stramenopiles, choanoflagellates and acantharians. We also found a novel lineage, closely related to dinoflagellates and not previously described.