The radioactive isotope ³²Si was used to measure Si uptake during a spring bloom dominated by the diatom Thalassiosira nordenskioeldii. The diatoms converted virtually all dissolved silicic acid in the surface layer into biogenic silica. The rate of Si uptake was unsaturated in all the experiments conducted during the bloom. The half-saturation constant for Si uptake declined from 6–12 mmol m^-3 early in the bloom to 2 mmol m^-3 when dissolved silicic acid concentration was low. The highest specific Si-uptake rates measured (ca. 0.3 d^-1 ) were comparable to average net growth rates calculated from cell abundance measurements.