The influence of gaseous environments on the growth of Monascus purpureus on prickly pear juice (PPJ) was investigated by varying the pO(2). Two fermentations were compared maintaining a low pO(2) (10%) for a short period during the first fermentation (FI) and a long period during the second fermentation (F2). The biomass increased in two stages corresponding to sugar consumption and ethanol formation, then to ethanol consumption with subsequent red pigment accumulation Red pigment formation started when glucose and fructose were completely assimilated and ethanol consumption began. The ethanol produced from sugars in F1 was 1.8 g/litre and the red pigments reached an absorbance (480 nm) of 6.18/ml at the end of fermentation. The ethanol accumulated in F2 was 7.6 g/litre and the final red pigment yield was 13.4/ml. The specific production rate of red pigments continuously increased and reached 0.08 g/h in F1 and 0.2 g/h in F2. These results are used to define an improved process based on PPJ for M. purpureus pigment production by the control of dissolved oxygen concentrations in order, in turn, to control ethanol concentration during the growth phase.