Organophosphorus esters of the formula (I), ##STR1## wherein R is a C.sub.1 -C.sub.2 alkyl group, R.sub.1 is a C.sub.1 -C.sub.2 alkyl, C.sub.1 -C.sub.2 alkoxy, C.sub.1 -C.sub.4 alkylamino, phenyl or phenoxy group, R.sub.2 and R.sub.3, which may be the same or different, are each a C.sub.1 -C.sub.5 alkyl or methoxy C.sub.2 -C.sub.3 alkyl group and X is an oxygen or sulfur atom, a method for producing organophosphorus esters of the formula (I) characterized by condensation reaction of s-triazines of the formula, ##STR2## wherein M is a potassium or sodium atom and R.sub.2 and R.sub.3 are as defined above, with a halogenated compound of the formula, ##STR3## wherein Y is a halogen atom and R, R.sub.1 and X are as defined above, and an insecticide, acaricide and/or nematocide characterized by containing organophosphorus esters of the formula (I) as an active ingredient.