The aim of the work was to find the optimal photon irradiance for the growth of green cells of Haematococcus pluvialis and to study the interrelations between changes in photochemical parameters and pigment composition in cells exposed to photon irradiances between 50 and 600 mu mol m(-2) s(-1) and a light: dark cycle of 12: 12 h. Productivity of cultures increased with irradiance. However, the rate of increase was higher in the range 50 - 200 mu mol(-2) s(-1). The carotenoid content increased with increasing irradiance, while the chlorophyll content decreased. The maximum quantum yield of PSII (F-v/F-m) gradually declined from 0.76 at the lowest irradiance of 50 mu mol(-2) s(-1) to 0.66 at 600 mu mol(-2) s(-1). Photosynthetic activity showed a drop at the end of the light period, but recovered fully during the following dark phase. A steep increase in non-photochemical quenching was observed when cultures were grown at irradiances above 200 mu mol(-2) s(-1). A sharp increase in the content of secondary carotenoids also occurred above 200 mu mol m(-2) s(-1). According to our results, with H. pluvialis green cells grown in a 5-cm light path device, 200 mu mol(-2) s(-1) was optimal for growth, and represented a threshold above which important changes in both photochemical parameters and pigment composition occurred.