TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Method: In the method for producing ethanol, the agitation means provides no more power than 0.2 watt/kg of total saccharification-fermentation mixture. The prokaryotic ethanologen is a member of a genus selected from Zymomonas, Zymobacter, Clostridium, Escherichia, Klebsiella, and Geobacillus. The agitation means comprises at least one impeller. The ethanologen of (c) is added at a time after addition of the saccharification enzyme of (b) when partial saccharification has occurred. Pretreated biomass is added in at least two portions that in combination give a total input insoluble solids concentration of at least 16% based on dry weight per liter. The concentration of total input insoluble solids in the saccharification-fermentation mixture is at least 20% based on dry weight per liter. The prokaryotic ethanologen is tolerant to the acetate concentration in the saccharification-fermentation mixture. Biomass is selected from switchgrass, waste paper, sludge from paper manufacture, corn cobs, corn husks, corn stover, grasses, wheat, wheat straw, hay, barley straw, rice straw, sugar cane bagasse, sorghum, components obtained from processing of grains, trees, branches, roots, leaves, wood chips, sawdust, shrubs and bushes, vegetables, fruits, flowers and animal manure. The pretreated biomass is produced by treating cellulosic biomass with ammonia, where the ammonia is less then 12 weight percent relative to dry weight of biomass. At least one saccharification enzyme is selected from cellulose-hydrolyzing glycosidases and hemicellulose-hydrolyzing glycosidases. At least one saccharification enzyme is a member of an enzyme consortium, where the saccharification enzyme consortium comprises enzymes selected from cellulases, endoglucanases, exoglucanases, cellobiohydrolases, beta -glucosidases, xylanases, endoxylanases, exoxylanases, beta -xylosidases, arabino-xylanases, mannases, galactases, pectinases, glucuronidases, amylases, alpha -amylases, beta -amylases, glucoamylases, alpha -glucosidases, isoamylases, peptidases, lipases, ligninases, and feruloyl esterases. The polysaccharides comprise xylan and glucan. The fermentable sugars comprise xylose and glucose. The concentration of ethanol produced is at least 40 g/L. Producing ethanol comprises (a) providing pretreated biomass of particle size equal to or less than 100 mu m or a particle size of equal to or more than 600 mu m, comprising insoluble solids and polysaccharides; (b) providing at least one saccharification enzyme for the conversion of polysaccharides to fermentable sugars; (c) providing a prokaryotic ethanologen; (d) preparing in a bioreactor comprising an agitation means a saccharification-fermentation mixture comprising the pretreated biomass of (a), the saccharification enzyme of (b), and the prokaryotic ethanologen of (c); and (e) growing the prokaryotic ethanologen in the saccharification-fermentation mixture where: (1) the concentration of total input insoluble solids in the saccharification-fermentation mixture is at least 16% based on dry weight per liter; and (2) where the prokaryotic ethanologen produces ethanol. Preferred Saccharification-Fermentation System: The biomass, enzyme, and ethanologen of (i) to (iii) are contained in a bioreactor comprising a functional agitation means which provides no more power than 0.2 watt/kg of total saccharification-fermentation mixture. EXAMPLE - No suitable example given.