Abstract. Armance MEA, Mattan-Moorgawa S, Bhagooli R. 2019. Micro-phytoplankton density and diversity at a pilot oyster culture barachois site of Mauritius Island. Ocean Life 3: 1-12. The density and diversity of marine micro-phytoplankton were studied at 4-6 stations from October to December 2014 within and around a pilot culture site of the oyster Crassostrea cuculata in the north-east of Mauritius Island to evaluate the water quality of the marine ecosystem suitable for oyster culture. Three micro-phytoplankton samples were taken at each station and the physicochemical parameters such as temperature, pH, and salinity were measured in situ while turbidity was measured ex-situ. No bloom was observed but a gradual increase in micro-phytoplankton density occurred from October to December 2014. The micro-phytoplankton density was positively correlated with temperature. A total of 20 genera of micro-phytoplankton were observed belonging to the classes of Baccillariophyta, Dinophyta, and Cyanophyta. The genera observed were Bacteriastrum, Chaetoceros, Coscinodiscus, Leptocylindrus, Cylindrotheca, Nitzschia, Pseudo-nitzchia, Asterionellopsis, Licmophora, Striatella, Flagilariopsis, Navicula, Pleurosigma, Thalassionema, Dinophysis, Gymnodinium, Alexandrium, Peridinium, Nitzschia, and Pleurosigma in the Bacillariophyta and in Dynophyta, Gymnodinium and Prorocentrum were the dominant genera, while only Lyngbya was observed as Cyanophyta. Simpson's diversity index revealed an increase in species diversity from October to December 2014 at studied stations. These findings indicate spatiotemporal variations in micro-phytoplankton density and diversity at the oyster culture site and further long-term studies are warranted to identify the optimal stations for oyster culture.