Competition partitioning experiments were conducted to determine the extent of shoot and root interference between sweet basil (Ocimum basilicum) and the weeds smooth amaranth (Amaranthushybridus) and livid amaranth (A.lividus). Sweet basil and amaranths were grown for 45 days in plastic 19-L containers filled with fertilized sandy soil. The plants were grown: 1) individually (one plant per container = no interference); 2) one basil plant and one amaranth plant together in the same container (= full interference); 3) one basil plant and one amaranth plant together in the same container, training the shoots apart to avoid canopy contact (= below ground interference); or 4) basil and amaranth grown in different containers set side by side (= above ground interference). Each basil/amaranth treatment was replicated five times and the experiment was conducted twice. The effects of smooth and livid amaranths on basil yield were the same for a given type of interference (full, above ground, below ground). Full interference from amaranth reduced basil shoot yield by about 35%, as compared to the yield of basil with no interference from amaranth. The effects of above-ground and below-ground interference on basil yield were additive, but interference above ground had a greater impact (about 21% basil yield loss) than below ground interference (about 14% basil yield loss). These results show that smooth and livid amaranths may drastically reduce sweet basil shoot yield, and that amaranth interference with sweet basil occurred to a greater extent above ground than below ground.