Pathogenicity of the white muscardine fungus, Beauveria bassiana (BALSAMO) VUILLEMIN, to a coreid bug, Riptortus linearis (F.), a pest of soybean, was investigated in a series of laboratory tests. Beauveria bassiana was pathogenic to 3rd, 4th and 5th stadium nymphs and adults of R. linearis. The 4th and 5th stadium nymphs were more susceptible than adults and 3rd instars. Within the range of 4.5×10³ to 4.5×10⁸ conidia/ml, there was a highly significant linear relationship between the dose of B. bassiana and the mortality of R. linearis. Isolates recovered from cadavers of R. linearis killed by B. bassiana were virulent to R. linearis. At 25°Cand above, pathogenicity of B. bassiana decreased with increases in temperature. This was due to the adverse effect of high temperatures on the germination of conidia. The longer the fungus-treated insects were maintained at high temperatures, the lower the insect mortality. Ultra-violet (UV) irradiation of conidia reduced pathogenicity of B. bassiana to R. linearis.