Abstract <p>A culture of the large diatom <i>Ditylum brightwellii</i> (T. West) Grunow isolated from the coastal waters of the Black Sea was grown under continuous low light (17.2 µmol photons m^–2 s^–1). The dynamics of its acclimation to high light intensities (172 and 425 µmol photons m^–2 s^–1) were studied. The process of short-term acclimation (within two hours) to 172 µmol photons m^–2 s^–1 was carried out mainly due to increase in non-photochemical quenching of chlorophyll fluorescence (NPQ) by 5 times after 90 min with its subsequent decrease and, to a lesser extent, due to a unidirectional increase in the C/Chl <i>a</i> ratio by 30% by the end of the second hour. Short-term acclimation to 425 μmol photons m^–2 s^–1 was achieved by a rapid (after 20&#xa0;min) eight-fold increase in non-photochemical quenching (NPQ) followed by a return to baseline values, intracellular chloroplasts’ aggregation within 50 min followed by a return to uniform distribution, a 50% increase in the C/Chl <i>a</i> ratio. Long-term acclimation (within 48 h) to both light intensities was accompanied by a 2.4–3-fold increase in the C/Chl <i>a</i> ratio and a 2-fold decrease in the maximal efficiency of photosystem II (<i>F</i>_v/<i>F</i>_m) and the proportion of active reaction centers (<i>RC</i>/<i>ABS</i>), and some variability of NPQ. The gradual formation of chloroplast conglomerates as an additional photoacclimation mechanism was observed throughout the entire period of long-term acclimation only at 425 µmol photons m^–2 s^–1.</p>