The efficiency of ethanol fermentation, as affected by grain source (maize and decorticated red sorghum), total sugar concentration (13 or 20A degrees Plato) and type of microorganism (Saccharomyces cerevisiae or Zymomonas mobilis) was studied. Maize mashes yielded 0.32 l ethanol kg(-1) ground grain whereas mashes prepared with decorticated red sorghum produced 0.28 l ethanol kg(-1). Both microorganisms yielded similar amounts of ethanol. However, high-gravity mashes (20A degrees Plato) yielded lower amounts of ethanol compared to counterparts adjusted to 13A degrees Plato (0.28 vs. 0.22 l ethanol kg(-1) ground grains). In decorticated sorghum mashes adjusted to 20A degrees P, Z. mobilis produced 40 ml kg(-1) more ethanol compared to S. cerevisiae. In addition, Z. mobilis had a lower dependency on nitrogenous compounds.