O-Alkyl-O-[1,6-dihydro-1-substituted-6-thioxopyridazin(3)yl]-(thiono) (thiol) phosphoric (phosphonic) acid esters and ester-amides of the formula ##STR1## in which R is alkyl with 1 to 6 carbon atoms,R' is alkyl, alkoxy, alkylmercapto or alkylamino with 1 to 6 carbon atoms per alkyl chain, or phenyl,R" is alkyl, cyanoalkyl, carbalkoxyalkyl or alkylcarbonylalkyl with 1 to 4 carbon atoms per alkyl chain, phenyl or phenyl carrying at least one substituent selected from the group consisting of halogen, nitro, alkyl with 1 to 4 carbon atoms and halogenoalkyl with 1 to 4 carbon atoms, andX is oxygen or sulfur,Which possess arthropodicidal properties.