Mixotrophic and heterotrophic growth of Haematococcus lacustris and rheological behaviour of the cell suspensions were investigated using batch culture in a 3.7 l fermenter. The microalga grew well on acetate in both growth modes, although growth was much better in mixotrophic culture than in the heterotrophic culture. The apparent viscosity of the cell-free medium of the two cultures was about 1 cp indicating that no viscous extracellular products such as polysaccharides were produced. In contrast, the apparent viscosity of the cell suspensions increased with increasing cell concentrations. The viscosity was affected not only by the cell density but also by the growth mode. In heterotrophic culture, the apparent viscosity increased steeply with cell concentrations above approx. 6.5 g l−1. In mixotrophic culture, however, the apparent viscosity increased markedly only when cell concentrations were higher than 25 g l−1. For both growth conditions, at low concentrations (i.e.
approx. 10 g l−1), the cell suspensions became non-Newtonian. The non-Newtonian characteristics of the culture fluids could be described by the Bingham plastic model. Both the yield stress and plastic viscosity increased with increasing cell concentrations.