Eggplant fruits are abundant in potassium, the amount of which ranges from 200 to 600 mg K⋅100 g-1 FM, depending on a variety. They are also a rich source of phosphorus, magnesium, calcium, and iron. As there are no fertilization recommendations for eggplant cultivation under cover, this study been undertaken to evaluate the vegetable’s requirements. The aim was to test how the type and dose of potassium fertilizer influences nitrogen, phosphorus, calcium, and magnesium levels in eggplant fruits. The experiment on cv. Epic F1 eggplant was carried out in unheated polyethylene tunnel in 2004-2005. The eggplant was cultivated on peat subsoil in 10 dm3 capacity cylinders made of rigid plastic. The experiment was set up in a two-factor, completely randomized design. The influence of two factors was examined: I – type of potassium fertilizer (KCl, K2SO4, KNO3), and II – potassium rate (8, 16, 24 g K⋅plant-1). Fruit samples for laboratory determinations were collected in mid-August, in the middle of fruiting stage. Fruits were harvested at the stage of technological maturity and the following were determined: Ntot, P, K, Ca, Mg. The results were processed by variance analysis. Significantly higher total nitrogen and potassium concentrations in fruits of plants fertilized with potassium nitrate as compared to the other two fertilizer types were recorded. Increasing potassium doses, regardless the fertilizer type, considerably increased the element content in eggplant fruits and widened the K:Ca ratio value. The diversification of potassium fertilization did not have significant influence on phosphorus and magnesium concentrations in eggplant fruits. No significant changes in calcium content in fruits were observed when applying potassium sulfate or nitrate, while higher potassium chloride rates significantly decreased the concentration of this element in fruits.