Novel O,S-dialkyl-O-(4-fluorophenyl)-(di)thiophosphoric acid esters of the formula ##STR1## where R.sup.1 is alkyl of from 1 to 3 carbon atoms, R.sup.2 is alkyl of from 1 to 5 carbon atoms, cycloalkyl of 5 or 6 carbon atoms, alkoxyalkyl of from 2 to 7 carbon atoms or alkylthioalkyl of from 2 to 7 carbon atoms, R.sup.3 is hydrogen, halogen, alkyl of from 1 to 4 carbon atoms, haloalkyl, alkoxy or alkylthio, each of from 1 to 4 carbon atoms, cyano or nitro, R.sup.4 denotes halogen, alkyl of from 1 to 4 carbon atoms, haloalkyl, alkoxy or alkylthio, each of from 1 to 4 carbon atoms, cyano or nitro, and X is oxygen or sulfur, R.sup.4 additionally denoting hydrogen when X is sulfur or when X is oxygen and R.sup.2 is methyl, n-propyl, sec-butyl, isobutyl, tert-butyl or pentyl, which compounds are valuable pesticides.