Biofilm cultivation of microalgae has great potential in many applications. However, the water footprint for this method has not been well assessed. This issue was explored with the microalga Haematococcus pluvialis. Only 1.25 l water is sufficient to support 1 m(2) biofilm cultivation surface. To produce 1 kg Haematococcus biomass and astaxanthin, the water footprint could be as low as 35.7 and 1440 l, respectively, by sealing the biofilm in a narrow chamber and supplying the proper amount of nutrients if the evaporation water loss was not considered. However, when loss of water by evaporation was considered, the water footprint was as low as 66.9 and 2700 l, respectively, if the chamber was aerated with CO2 at 0.014 vvm. These water footprint values are much lower than values obtained in other research work. The water footprint of biofilm microalgal cultivation can be potentially reduced by more than 90 % if the biofilm is sealed in a narrow chamber and supplied with a slow aeration of CO2 as carbon source.