A new species, Phomopsis cucurbitae, is described as the chief causal agent of a black rot disease of greenhouse cucumbers in Ontario. In addition to causing stem canker, the fungus occasionally causes decay of fruits. Similarities in macroscopic symptoms caused by P. cucurbitae and by another fungus, Mycosphaerella melonis, that also causes black rot of cucurbits, are pointed out. The pathogenicity of P. cucurbitae on cucurbits and tomato is discussed.