Acid-resistant specimens of trilobites were recovered from the dissolved residues of lime mudstone collected from the lower part of the Machari Formation, Yongwol Group, Taebaeksan Basin, Korea. One of the samples yields a trilobite assemblage comprising Lejopyge armata (Linnarsson, 1869), Lisogoragnostus coreanicus n. sp., agnostid genus and species indeterminate, ammagnostid genus and species indeterminate, clavagnostid genus and species indeterminate, Cyclolorenzella sp., and Eoshengia? sp. The stratigraphic interval yielding the trilobite assemblage is herein designated as the Lejopyge armata Zone, which is correlated well with the upper Middle Cambrian biozones established elsewhere. This provides the first biostratigraphic reference point for the upper Middle Cambrian sequence of the Yongwol Group and fills partially the biostratigraphic gap between the Tonkinella (middle Middle Cambrian) and the Glyptagnostus stolidotus (uppermost Middle Cambrian) zones in the Machari Formation.