The kinetics and regulation Of D-Xylose uptake were investigated in the efficient pentose fermentor Candida succiphila, and in Kluyveromyces marxianus, which assimilate but do not ferment pentose sugars. Active high-affinity (K-m similar to 3.8 mM; V-max similar to 15 nmol/[mg.min]) and putative facilitated diffusion low-affinity (K-m similar to 140,mM; V-max similar to 130 nmol/ [mg.min]) transport activities were found in C. succiphila grown, respectively, on xylose or glucose. K. marxianus showed facilitated diffusion low-affinity (K-m similar to 103 mm; V-max similar to 190 nmol/ [mg.min]) transport activity when grown on xylose under microaerobic conditions, and both a low-affinity and an active high-affinity (K-m similar to 0.2 mM; V-max similar to 10 nmol/ [mg.min]) transport activity when grown on xylose under fully aerobic conditions.