Stem canker caused by Diaporthe phaseolorum (Cke. & Ell.) Sacc. var. caulivora Athow & Caldwell is a serious disease of soybeans in Ontario. Artificial inoculation experiments have indicated (a) that soybeans which are highly susceptible to infection at midstage in their development become progressively less so as they approach maturity, (b) that reduction in yield is more or less directly proportionate to earliness of infection, (c) that all varieties presently of commercial importance in Ontario are highly susceptible to the disease, and (d) that six common weeds, and tomato, pepper, and snap and field bean, are not susceptible to infection by the soybean pathogen. The tests also indicated that monoconidial isolates of the stem canker organism are as highly pathogenic as those derived from single ascospores, and that isolates, though morphologically similar, may be differentiated on the basis of their pathogenic capability. The pathogen is definitely seedborne, but infection from such a source is not of primary importance. The likelihood that the pathogen is indigenous to the soil of the soybean-growing area of Ontario is discussed. Pod and stem blight caused by Diaporthe phaseolorum (Cke. & Ell.) Sacc. var. sojae is of negligible economic importance in Ontario. The pathogen, which is observed mostly in the imperfect (Phomopsis) stage, shows little evidence of being able to infect plants until late in the growing season. In the absence of resistant varieties no better methods of control can be recommended than those of completely plowing under soybean debris, of avoiding the planting of the more susceptible varieties, and of practicing crop rotation.