A 3-acyl-2-nitromethylene-tetrahydro-2H-1,3-thiazines of the general formula: ##STR1## in which R represents an optionally ring-substituted benzyl group; a cycloalkyl group optionally substituted by halogen and/or alkyl; or an alkyl, alkenyl or alkynyl group of 2 or more carbon atoms optionally substituted by one or more of the same or different substituents selected from halogen, optionally substituted phenyl, phthalimido, alkoxy, haloalkoxy, alkoxyalkoxy, hydroxy, thiol, cycloalkyl optionally substituted by halogen and/or alkyl, and groups of the formula--COR.sup.1, --X.CO.R.sup.1 or --CO.X.R.sup.1