The objective of this work was to estimate the disappearance of pyrimethanil the active ingredient of Mythos 300 SC. At present, Mythos 300 SC is commonly used for the protection of greenhouse vegetables against diseases of fungal origin. The behaviour of pyrimethanil deposits was studied on tomato plants grown in commercial greenhouses sprayed with homogenous 0.15% aqueous solution of this plant protection product. It was found that the value of pyrimethanil residues on ripening fruits decreased by half and reached zero levels in 5.7 and 13.7 days after Mythos 300 SC application, respectively. Pyrimethanil residues on tomato leaves decreased by half within 4.0 days and reached zero level in the first 10.5 days. Therefore, in conditions of high infection pressure, there is a need to repeat the fungicide application as early as after 3 or 4 days after previous application of Mythos 300 SC.