To conserve energy in the production of astaxanthin by the green alga Haematococcus pluvialis, we utilized intermittent flashing light from blue light emitting diodes (LEDs) and investigated the effects of the incident light intensity (2-12 mu mol m(-2) s(-1)), duty cycle (17-67%) and frequency (25-200 Hz) of flashing on the cell growth and astaxanthin production. In the above ranges, the final astaxanthin concentration under illumination by flashing light was significantly higher than that obtained under illumination with continuous light at the same incident intensity. For example, flashing light at an incident intensity of 8 mu mol m(-2) s(-1) gave the same final astaxanthin concentration that was obtained under continuous light illumination at 12 mu mol m(-2) s(-1), thus reducing energy consumption by 1/3. We therefore conclude that flashing light from blue LEDs is a promising illumination method for indoor algal cultivation using photobioreactors.