Green vegetative cells of Haematococcus pluvialis grew heterotrophically on acetate in the dark. When vegetative cells were stressed with 0.1% NaCl, MSCl2, KCl or CaCl2, cyst formation was induced rapidly in the dark. Salt-stressed, 8-day old brown-red cyst cells in the dark contained astaxanthin at 30 pg/cell (9 mu g/ml). It was concluded that salt stress causes both cyst formation and carotenoid biosynthesis in the dark.