Abstract: Biological control of sugar beet damping-off of Rhizoctonia solani by two Streptomyces isolates (S2 and C) was evaluated in this study. The in vitro antagonism assays showed that active isolates had inhibitory effects on mycelium growth of the three R. solani AG-4 isolates (Rs1, Rs2 and Rs3). Soil treatment either with isolate S2 or C formulation inhibited the disease completely and increased seedling stand in infected and uninfected treatments significantly (p<0.05). Compared to controls, all treatments containing bacteria had enhanced shoot and root dry weight and root density. Both bacterial isolates maintained normal growth in pH ranges of 5.6, 7.2 and 8.0 at 29°C. Isolate C grew in pH ranges 5.6, 7.2 and 8.0 at 23-37°C, isolate S2 grew at 18-37°C in pH 5.6-8.0 but did not grow in acidic medium at 37°C. When pH decreased to 5.0, growth of C decreased and S2 inhibited. To elucidate the mode of antagonism, chitinase activity and siderophore production were evaluated. Both isolates showed chitinase activity on medium containing colloidal chitin. Biosynthesis of siderophore was detectable in isolate C but not in S2. The results of this study showed that these isolates had antifungal activities by production of siderophore and chitinase.