The present invention relates to novel pyrimidine acyclonucleoside derivatives represented by the following general formula (I), antiviral agents containing the derivatives as the active ingredients and processes of preparation thereof. ##STR1## wherein R.sup.1 is ethyl or isopropyl; R.sup.2 is (3,5 dimethylphenyl)selenenyl; R.sup.3 is phenyl or methyl; X is oxygen; and Y is oxygen.