Morphological and genetic traits of Lingula adamsi were compared among individuals from South Korea (Yellow Sea) and Japan (Ariake Sea). The Japanese population is usually identified as L. shantungensis, although this nominal species has been placed in synonymy with L. adamsi. Some studies have suggested that the Japanese population is a different species or subspecies from L. adamsi based on morphological comparisons. In this study, morphological characters of the shell of ten individuals of L. adamsi collected from southwestern South Korea, such as shape, color, and surface ornamentation, were compared with those of the Japanese population. Although the width/length ratio of the dorsal shell is significantly different between the South Korean and Japanese samples, this and all other morphological differences can be considered as intraspecific variation. The nucleotide sequences of the mitochondrial coxl gene region indicate a very low level of variation between the ten South Korean individuals and an individual from the Ariake Sea, Japan, not greater than that observed within local populations of Lingula anatina. The forms from South Korea and Japan, therefore, are best interpreted as conspecific from the viewpoint of both morphological and genetic variation.