The marine diatom Haslea ostrearia was cultured under light of different qualities, white (WL), blue (BL), green (GL), yellow (YL), red (RL), and far-red (FRL) and at two irradiance levels, low and high (20 and 100 micromolphotonsm(-2)s(-1), respectively). The effects of the different light regimes were studied on growth, pigment content, and photosynthesis, estimated by the modulated fluorescence of chlorophyll, as relative electron transport rate (rETR). For all the light qualities studied, growth rates were higher at high irradiance. Compared to the corresponding WL controls, growth was higher in BL and lower in YL at low irradiance, and lower in YL and GL at high irradiance. Except for YL, almost all the pigment contents of the cells were lower at high irradiance. At low irradiance, cell pigment contents (chlorophyll a and c, fucoxanthin) and pigment ratios (in function of chlorophyll a) were lower in YL, RL, and FRL. Whatever the irradiance level, the maximum PSII quantum efficiency (F(v)/F(m) remained almost constant for WL, BL, and GL. Other fluorescence parameters (photochemical quenching, rETR(max), and alpha, the maximum light utilization coefficient) were lower in GL, YL, RL, and FRL, at low irradiance. Although not statistically significant, BL caused an increase in these fluorescence parameters. These findings are interpreted as evidence that inverse chromatic acclimation occurs in diatoms.