The hypothesis of Black, Chen and Brown that C4 plants are better competitors than C3 plants, and that most weeds in summer crops have C4 photosynthesis, is examined. With reference to the eastern United States, it is concluded that both C3 and C4 weeds are poor competitors, many noxious weeds of cultivated crops have C3 photosynthesis, and in general C4 photosynthesis is less important than other features of plants in determining their growth rate and competitive ability. Even in desert plant communities, C4 species do not appear to be competitively superior to C3 species.