O-�1-substituted-6-pyridazinon(3)yl!-thionoalkanephosphonic acid esters of the formula ##STR1## in which R.sub.1 and R.sub.2 each independently is alkyl with 1 to 7 carbon atoms, R.sub.3 is alkyl, cyanoalkyl, alkylcarbonylalkyl, carbalkoxyalkyl, hydroxyalkyl, alkylthioalkyl or halogenalkyl with 1 to 4 carbon atoms per alkyl radical, alkenyl or alkynyl with 3 to 5 carbon atoms, benzyl, benzyl carrying at least one substituent selected from halogen and alkyl with 1 to 3 carbon atoms, phenyl, phenyl carrying at least one substituent selected from nitro, halogen, alkyl with 1 to 3 carbon atoms and carbalkoxy with 1 to 4 carbon atoms, morpholinomethyl or piperidinomethyl,R.sub.4 and R.sub.5 each independently is hydrogen or alkyl with 1 to 3 carbon atoms, and X and Y each independently is oxygen or sulfur, Which possess insecticidal and acaricidal properties.