Comprehensive studies of pure colonies of Streptomyces fradiae in the production of xylose isomerase by submerged fermentation at shake flask level revealed poor culture stability with respect to enzyme production, biomass formation, degree of pigmentation, quantity of glucose and xylose utilization, level of enzyme in cell-free culture broth and final pH of the fermentation medium. The results serve to stress obligatory evaluation of culture stability of Streptomyces strains in determining their suitability for use in developing fermentation processes for commercial exploitation.