The reactions of ethyl monofluoromalonaldehyde potassium enolate (1) and S-n-alkylisothiourea (2a-e) gave 2-n-alkylthio-5-fluoro-4 (1 or 3H)-pyrimidinones (3a, d, e), 2-n-alkylthio-5-fluoro-6-monofluoromethyl-4 (1 or 3H)-pyrimidinones (4a-c) and 2-n-alkylthio-6-n-alkylthiomethyl-5-fluoro-4 (1 or 3H)-pyrimidinones (5a-e).