NOVELTY - Producing ethanol comprises preparing, in a bioreactor comprising an agitation means, a saccharification-fermentation mixture comprising the pretreated biomass, saccharification enzyme, and prokaryotic ethanologen; and growing the prokaryotic ethanologen in the saccharification-fermentation mixture.    USE - The method and system are used for producing ethanol (claimed).    ADVANTAGE - The present invention provides the use of a prokaryotic ethanologen in simultaneous saccharification and fermentation (SSF) processes via identifying conditions that allow for the use of high input insoluble solids content in the saccharification and fermentation mixture, and that support production by the prokaryotic ethanologen such that high ethanol production is achieved.    DETAILED DESCRIPTION - Producing ethanol comprises (a) providing pretreated biomass 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 (d) growing the prokaryotic ethanologen in the saccharification-fermentation mixture where, the concentration of total input insoluble solids in the saccharification-fermentation mixture is at least 16% based on dry weight per liter, and where the prokaryotic ethanologen produces ethanol. An INDEPENDENT CLAIM is a saccharification-fermentation system comprising (i) a pretreated biomass comprising insoluble solids and polysaccharides, (ii) at least one saccharification enzyme for the conversion of polysaccharides to fermentable sugars; and (iii) a prokaryotic ethanologen; where the biomass, enzyme, and ethanologen of (i), (ii) and (ii) are combined in a saccharification-fermentation mixture, having a concentration of total input insoluble solids that is at least 16% based on dry weight per liter.