2-(Pyrimidyl or pyrazinyl)-imino-1,3-dithietanes ##STR1## wherein Ar is one of the radicals ##STR2## R is unsubstituted C.sub.1 -C.sub.3 alkyl or C.sub.1 -C.sub.3 alkyl which is substituted by halogen, C.sub.1 -C.sub.3 alkyl, C.sub.1 -C.sub.3 alkoxy, C.sub.1 -C.sub.3 alkylthio or cyano, independently of one another, or is unsubstituted or halo-substituted C.sub.3 -C.sub.7 alkenyl, unsubstituted or halo-substituted C.sub.3 -C.sub.7 alkynyl, or C.sub.3 -C.sub.5 cycloalkyl, C.sub.1 -C.sub.3 alkoxy, C.sub.1 -C.sub.3 alkylamino, nitro or halogen,n is a number from 0 to 3,X.sub.1 and X.sub.2 independently of each other are hydrogen or halogen, including the addition salts formed with acids of the formula H.sup..sym. X.sup..crclbar. in which the anion X.sup..crclbar. is Cl.sup..crclbar., Br.sup..crclbar., I.sup..crclbar., HSO.sub.4 -, H.sub.2 PO.sub.4 -, H.sub.2 PO.sub.3 -, NO.sub.3 -, CH.sub.3 COO.sup..crclbar., CH.sub.2 ClCOO-, CF.sub.3 COO-, HOCH.sub.2 COO-, CH.sub.3 CH(OH)COO.sup..crclbar., HOOCCOO.sup..crclbar., HOOCCH.sub.2 COO.sup..crclbar., HOOCCH.dbd.CHCOO.sup..crclbar., ##STR3## a process for the preparation of the compounds of formula I, and novel intermediates of the preparation process are described.The compounds of formula I have nematicidal and fungicidal properties. Nematicidal and fungicidal compositions that contain at least one compound of formula I as active ingredient, and also methods of using the compounds and the compositions in the control of nematodes and fungi, are described.