Production of biomass and phycocyanin (PC) were investigated in highly pigmented variants of the unicellular rhodophyte Galdieria sulphuraria, which maintained high specific pigment concentrations when grown heterotrophically in darkness. The parental culture, G. sulphuraria 074G was grown on solidified growth media, and intensely coloured colonies were isolated and grown in high-cell-density fed-batch and continuous-flow cultures. These cultures contained 80-110 g L-1 biomass and 1.4-2.9 g L-1 PC. The volumetric PC production rates were 0.5-0.9 g L-1 day(-1). The PC production rates were 11-21 times higher than previously reported for heterotrophic G. sulphuraria 074G grown on glucose and 20-287 times higher than found in phototrophic cultures of Spirulina platensis, the organism presently used for commercial production of PC.