A freshwater ecto-parasitic crustacean was isolated from the naturally infested goldfish (Carassius auratus) purchased from commercial aquarium in Korea. Based on the morphological comparison with all Argulus species, isolated fish lice was identified as Argulus japonicus Thiele, 1899. A. japonicus is native to Asia, which also is a habitat for its typical hosts, such as goldfish and common koi carp (Cyprinus carpio). A. japonicus has not previously been described from goldfish from the ornamental facilities in Korean. This is the first report of A. japonicus morphology with light and scanning electron microscopy.