A two-factorial field experiment was performed in a randomized block design with three replications in 2010-11 in the Experimental Garden of the University of Warmia and Mazury in Olsztyn, to determine the effect of seedling planting time on the concentrations of mineral compounds in basil leaves. Plot area was 1.2 m². The experimental factors were: (1) plants of sweet basil, purple (purpurascens) basil, and cinnamon basil, and (2) time of planting basil seedlings. The concentrations of macroelements (total nitrogen, phosphorus, potassium, magnesium, calcium, and sodium) and microelements (iron and copper) were determined in basil leaves. The content of total nitrogen, phosphorus, potassium, magnesium, calcium, and sodium in basil leaves depended on the cultivar. Cinnamon basil contained the highest concentrations of total nitrogen, phosphorus, magnesium, and calcium, whereas sweet basil had the highest content of potassium and sodium. The time of planting seedlings had a significant effect on the iron content of basil leaves, which was higher in plants that developed from seedlings planted on 19 May.