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Abstract: Fractionation of silicon (Si) isotopes was measured in seven species (nine strains) of polar and sub-polar marine diatoms grown in semi-continuous unialgal cultures under optimal irradiance and temperature for each diatom strain. Results from this work provide the first evidence that Si isotope fractionation by diatoms is species-dependent. The greatest difference in the Si isotope fractionation factor (ε) was observed between two Southern Ocean diatoms, Fragilariopsis kerguelensis (−0.54‰, average for two strains) and Chaetoceros brevis (−2.09‰). The ε for the other species, both polar and sub-polar, ranged from −0.72‰ to −1.21‰. The two remaining polar diatoms had ε values of −0.74±0.05‰ for Thalassiosira antarctica, and −1.21±0.04‰ for Thalassiosira nordenskioeldii, while the sub-polar species had ε values of −0.72±0.04‰ for Thalassiosira weissflogii, −0.88±0.06‰ for Thalassiosira pseudonana (CCCM58), −0.97±0.14‰ for Thalassiosira pseudonana (CCMP1014), and −1.15±0.03‰ for Porosira glacialis. The range in ε for the diatoms evaluated in this study may be large enough to significantly impact the Si isotope composition measured in diatom opal (δ30Si-bSiO2) from marine sediments and its subsequent interpretation. To test the influence of diatom taxonomic composition on δ30Si-bSiO2, we developed a model that considered the relative abundance of diatom species and the ε values (from this study) for each species present within the sediment core (i.e. weighted-average ε). The model was applied to records from a Southern Ocean sediment core (TN057-13) where both diatom abundance and δ30Si-bSiO2 data were available. The analysis indicated that 67% of the variation in δ30Si-bSiO2 could be explained by species-dependent Si isotope fractionation. We suggest that future work should assess phytoplankton taxonomic composition when using δ30Si-bSiO2 as a proxy for Si utilization. [Copyright &y& Elsevier]