The characteristics of the phytoplankton of Takapoto Atoll are reviewed from the studies conducted between 1974 and 1998. These studies mainly concerned the biomass and primary production of phytoplankton while the taxonomic composition received far less attention. The mean biomass is 0.2–0.3 μg chl a·L–1. The phytoplankton is homogeneously distributed on a year scale although an higher biomass (0.8 μg chl a·L–1) may temporarily exist in the south part of the atoll under moderate tradewinds or calm weather. The gross primary production reached 0.8 g C·m–2·day–1 whereas the net primary production is estimated to be 0.7 g C·m–2·day–1. No significant long-term changes of the biomass or primary production can be observed. The implications of this stability are discussed in the context of the mother-of-pearl mariculture. Size fractionated samples revealed the predominance of picophytoplankton which represented more than 60% of the phytoplankton biomass and achieved > 50% of the primary production. The taxonomic composition observed in 1974 showed the predominance of three algal groups: diatoms, dinoflagellates and coccolithophorids. The diatoms were the most diversified group, while the dinoflagellates were the most abundant. No further examination of the phytoplankton was undertaken until 1996. At that time, the microplankton was quite absent, and the phytoplankton communities were dominated by the pico- and nanophytoplankton, mainly chlorophytes, prymnesiophytes and dinoflagellates. This drastic shift of the phytoplankton communities towards smaller size is not clearly understood. It emphasises the need of taxonomic studies for a better understanding of the lagoon ecology.