NOVELTY - Propagating ethanologen comprises: (a) providing a medium for propagation of ethanologen; (b) supplying a first cell mass of ethanologen to the medium; (c) supplying xylose to the medium as a carbon source for cell mass growth of the ethanologen; (d) maintaining the medium at a pH of 5-6 and at a temperature of 26-37 degrees C, so that the first cell mass is propagated into a second cell mass of ethanologen, under aerobic conditions with an airflow of at least 1 volume of air per volume of medium per minute, where the second cell mass of ethanologen is larger than the first cell mass of ethanologen.    USE - The method is useful for propagating ethanologen that is used for production of ethanol (claimed).    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a system for propagating ethanologen for production of ethanol from biomass in fermentation system, comprising: a first stage comprising a first vessel configured to maintain the medium comprising ethanologen; a second stage comprising a second vessel configured to maintain the medium supplied from the first stage; a source of xylose to be provided to the medium as a carbon source for ethanologen in the first stage; a source of xylose to be provided to the medium as a carbon source for the ethanologen in the second stage, where the ethanologen has a first cell mass supplied to the first stage and a second cell mass supplied from first stage to the second stage, and third cell mass supplied from the second stage, the medium in the first vessel is maintained at the pH of 5-6 and at 26-37 degrees C with the airflow, so that ethanologen can be propagated into the second cell mass, the medium in the second vessel is maintained at the pH of 5-6 and 26-37 degrees C with the airflow so that ethanologen can be propagated into the third cell mass, the third cell mass is at least 20 times larger than the second cell mass, and the second cell mass is at least 200 times larger than the first cell mass.