A study of the competitive relationship between Johnson grass - Sorghum halepense (L.) Pers and mid-early and late varieties of potatoes was conducted during the period 2009-2011 in order to establish the extent of the harmful effects of the weed on the crop. A two-factor field experiment was done applying the method of fractional parcels with four varieties of potatoes and a different density of Johnson grass - from 0 to 8 per m2 in a geometric progression. The potato production grown without Johnson grass competition was the highest and ranged from 1414.3 kg/dka for the Agria variety to 2367.3 kg/dka for the Markiz variety. The yield of potatoes with a density of 8 per m2 of Johnson grass proved the lowest. The relatively high yields under the experimental conditions were due to the fact, that the experimental plots were kept weed-free throughout the growing season of the crop.