New phenylthionophosphonic acid O,O-di-esters of the general formula: ##STR1## wherein R.sub.1 denotes a lower alkyl group; R.sub.2 denotes a hydrogen atom, a lower alkyl group or an unsaturated lower alkyl group; and R.sub.3 denotes a lower alkyl group or an unsaturated lower alkyl group are provided, which have high insecticidal, miticidal and nematocidal activities and are useful as pesticidal agents to be applied to insect, acarine and/or nematode pests.