This research examined the titers of hexokinase (HK), phosphofructokinase (PFK), and xylulokinase (XUK) in Saccharomyces cerevisiae and two xylose fermenting yeasts, Pachysolen tannophilus and Candida shehatae, following shifts in carbon source and aeration. Xylose-grown C. shehatae, glucose-grown P. tannophilus, and glucose-grown S. cerevisiae, had the highest specific activities of XUK, HK, and PFK, respectively. XUK was induced by xylose to moderate levels in both P. tannophilus and C. shehatae, but was present only in trace levels in S. cerevisiae. HK activities in P. tannophilus were two to three fold higher when cells were grown on glucose than when grown on xylose, but HK levels were less inducible in C. shehatae. The PFK activities in S. cerevisiae were 1.5 to 2 times higher than in the two xylose-fermenting yeasts. Transfer from glucose to xylose rapidly inactivated Wt in P. tannophilus, and transfer from xylose to glucose inactivated XUK in C. shehatae. The patterns of induction and inactivation indicate that the basic regulatory mechanisms differ in the two xylose fermenting yeasts.