Straw has the highest concentration of the natural radioisotope40K in comparison with other biofuels and as peat, wood, wood chips, and energy forest, Concentration of40K have been measured in the ash remaining after wheast straw was burnt in a water heating plant. Samples wre also taken of the wheat ears, straw, and surrounding earth. The highest concentration of40K found in the bottom ash was 6000 Bq/kg which is about twenty times higher than the activity concentration of40K in the straw and about ten times higher than the natural40K activity in the ground. Calculations of the maximum ground levels air concentration of40K using the Gaussian plume model give a value of a few hundred Bq/m3. This value is found at a distance of only 100 m from the plant because of the short stack height.