Long-term (149 d) continuous fermentation was used to adapt a xylose-fermenting recombinant Zymomonas mobilis, strain 39676:pZB4L, to conditioned (overlimed) dilute-acid yellow poplar hemicellulose hydrolyzate ("prehydrolyzate"). An "adapted" variant was isolated from a chemostat operating at a dilution rate of 0.03/h with a 50% (v/v) prehydrolyzate, corn steep liquor, and sugar-supplemented medium, at pH 5.75. The level of xylose and glucose in the medium was kept constant at 4% (w/v) and 0.8% (w/v), respectively. These sugar concentrations reflect the composition of the undiluted hardwood prehydrolyzate. The level of conditioned hardwood prehydrolyzate added to the medium was increased in 5% increments starting at a level of 10%. At the upper level of 50% prehydrolyzate, the acetic-acid concentration was about 0.75% (w/v). The adapted variant exhibited improved xylose-fermentation performance in a pure-sugar, synthetic hardwood prehydrolyzate medium containing 4% xylose (w/v), 0.8% (w/v) glucose, and acetic acid in the range 0.4-1.0% (w/v). The ethanol yield was 0.48-0.50 g/g; equivalent to a sugar-to-ethanol conversion efficiency of 94-96% of theoretical maximum. The maximum growth yield and maintenance energy coefficients were 0.033 g dry cell mass (DCM)/g sugars and 0.41 g sugars/g DCM/h, respectively. The results confirm that long-term continuous adaptation is a useful technique for effecting strain improvement with respect to the fermentation of recalcitrant feedstocks.