Induction of oxidative stress by UVA and UVB in the diatom Thalassiosira pseudonana was experimentally studied. Cells, pre-grown in a light-limited continuous culture, were incubated for 4 h at 175 µmol m-2s-1photosynthetically active radiation, with optional supplementary UVA at an unweighted dose rate of 0.70 W m-2, or 2.79 W m-2UVA plus 0.45 W m-2UVB (unweighted). A fluorescence-based measure of photosynthetic efficiency (Fv/Fm) decreased from 0.69 to 0.58 in the presence of UVB, whereas UVA caused a minor decrease of Fv/Fm. Quantitative analysis of confocal images showed a minor increase of active oxygen production associated with supplemental UVA alone, and a 100% increase with additional UVB. Cellular malondialdehyde, an indicator of lipid peroxidation by active oxygen, almost doubled under UVA and increased three-fold with additional UVB. Activities of superoxide dismutase (scavenging active oxygen) and glutathione reductase (reducing GSSG to GSH) increased in response to UVB exposure, whereas ascorbate peroxidase activities did not. UVB caused a minor decrease in the glutathione ratio GSH : (GSH + 0.5GSSG), which indicates a moderate oxidative stress.