Xylone isomerase (D-xylose ketol-isomerase, EC 5,3,1,5) have been demonstrated in the cell-free extracts of Stroptomuces canus and Streptomuces malachiticus grown in the presence of xylose. Xylose, glucose and ribose served as substrates for the enzymes of the two strains with respective $K_m$ values of 22, 130, 290 mM (S. canus) and 7,83,637 mM(S.malachiticus), and $V_max$ values of 1,000, 0.087, $.0222{mu}moles/min/mg$ protein (S. canus) and 0.312, 0.083, 0.500.$mu$moles/min/mg protein (S. malachiticus). L-Rhammose was also isomerized by the crude enzyme solutions of the two strains. The maximal activities of the two xylose-isomerases were observed at pH 7.5 and $75^{circ}C$. The xylose isomerase activities of the two strains were activated two-three times by $Mg^{++};and;Co^{++}$ as that of control, partially activated by $Ba^{++}$ and inhibited by $Ni^{++},;Ca^{++};and;Zn^{++}$. Particulary, the addtion of $Mn^{++}$ stimulated xylose-isomerizing activities, but inhibited glucose-isomerizing activities in both strains. However, $Cu^{++}$ inhibited xylose-isomerizing activities, while stimulated glucose-isomerizing activities of the enzymes.