The continuous fermentation of D-xylose to ethanol by free cells of Clavispora opuntiae was studied in a continuous stirred tank reactor at 25 +/- 0.5 degrees C and pH 4.2 +/- 0.2. The maximum 89.8 +/- 0.04% ethanol yield was obtained as compared to theoretical yield and about 96.00 +/- 0.05% sugar was utilized at a dilution rate of 0.014 h(-1). The maximum values of volumetric ethanol (Q(p)) and biomass (Q(x)) productivity were found to be 0.67 +/- 0.04 g(p) l(-1) h(-1) and 0.18 +/- 0.007 g(x) l(-1) h(-1), respectively, at a dilution rate of 0.09 h(-1). The maintenance energy coefficient for C. opuntiae was found to be 0.21 +/- 0.005 g(s) g(x)(-1) h(-1), whereas the values of specific ethanol production rate (q(p)) and specific sugar uptake rate (q(s)) were found to increase almost linearly with the increase of dilution rate. The maximum specific ethanol productivity (qp max) and maximum specific sugar uptake rate (q(s) (max)) were 0.50 +/- 0.004 g(p) g(x)(-1) h(-1) and 4.00 +/- 0.006 g(s) g(x)(-1) h(-1), respectively.