The content of 40K natural radionuclide in bananas (Musa sapientum) from the Vale do Ribeira region, Sao Paulo, Brazil, has been measured. We have collected several samples of bananas prata and nanica, its peels, leaves, and also different soils where the banana tree was planted, such as soil with a standard amount of fertilizer, the fertilizer itself and also soil without fertilizer for comparison. We have used the gamma‐ray spectroscopy technique with a NaI(T1) crystal inside a 12 cm thick lead shield to detect the gamma‐radiation. The results indicate that only part of the available potassium is absorbed by the plant, which is mainly concentrated in the banana peel.